How was it decided what the order of the letters in the alphabet would be in?
Okay, so there is an alphabet section in the FAQ's but I wasn't satisfied with the answers there which mostly boiled down to 'Romans/Greeks/Phonecians did it' so I wrote out a more detailed answer when this was asked a few months ago:
Let's start from the beginning... Our alphabet has a history spanning over 3000 years, its origins trace back to the earliest deciphered alphabetic writing system 'North Semitic' which began in the middle east around 1700 BC. We don't know exactly which people invented NS, but possible candidates include early Canaanites, Hebrews and Phoenician as the languages of these groups at this time were very closely related, in effect they were dialects rather than separate languages.
The North Semitic alphabet is more usually referred to simply as 'the Phoenician alphabet' due to the fact that it was the Phoenicians who are credited with introducing the concept to the Ancient Greeks, but 'North Semitic' is a more accurate name as we don't know for sure which North Semitic people used it first.
A common convention is to use 'Phoenician alphabet' after 1200 BC (by which time we know the Phoenicians were using it in its established form) and 'Proto-Canaanite' or 'Proto-Sinaitic for the earlier developmental stages of the alphabet. PC/PS were not fully alphabetic systems themselves but rather were bridging systems between the earlier pictographic writing systems and the emerging alphabetic systems.
Going further back we can see signs of an alphabetic system developing in Mesopotamian cuneiform and Egyptian hieroglyphs. This suggests that rather than having a single source early alphabet systems developed through interactions between cultures. For example being able to write phonetically (according to sounds) would be useful if you needed to record the name of a visiting foreign dignitary. It also makes trade more efficient if things can be recorded quickly and accurately.
By the 11th century BC we have a fully formed Phoenician alphabet with 22 letters. Unlike our modern alphabet this was an abjad, which means that only consonant sounds were written, the reader would need to use their own knowledge of the language to determine what the vowel sounds should be. This consonant 'abjad' system also included letters to indicate a glottal stop and a voiced pharyngeal approximant. Without getting too technical those are simply sounds found in some languages and dialects (the glottal stop in some English dialects) which we do not have letters for.
The Phoenicians were famed sea-faring traders and during the 8th c BC they brought their abjad system to the Greeks, who adapted it by adding vowel signs. Those two symbols for a glottal stop and a voiced pharyngeal approximant were converted to express vowel sounds, since ancient Greek did not need signs for those sounds and two new letters were also added. Giving a new total of 24.
This story is repeated often in the history of the alphabet, with speakers of different languages and dialects changing the sounds represented by letters or adding and subtracting letters in order to make it better fit the sounds they use.
Several different forms of the Greek alphabet emerged in different Greek speaking regions, one of these 'Euboean' is suggested as the likeliest candidate for the transmission of this alphabetic system to the Etruscans.
The alphabets used by the Etruscans and other pre-Roman peoples of the region are known as 'old Italic'. There were several co-existing forms but it is specifically the Etruscan one which is credited with being the direct ancestor of the Latin alphabet. Etruscan had 26 letters at the time that the Romans began adopting it, but only 21 of these found their way to Latin.
The Roman conquest of Greece in the 1st c. BC led the Romans to adopt two new letters /Y/ and /Z/ from the Greek alphabet in order to write Greek loan words. So now we have 23 letters. Despite of the influence of Roman culture the Germanic peoples (the Angles, Saxons and Jutes) who would arrive in England starting from the mid 5th c. to eventually become 'the English' after the departure of the Roman armies from Britain did not adopt the Roman/Latin alphabet for Anglo-Saxon/Old English but instead continued to use their Runic alphabets.
The origins of these Runic alphabets is somewhat mysterious, they probably have their origins in the Old Italic alphabets mentioned as they emerge during times of contact between Germanic peoples and the expanding Roman Empire, with exchanges such as German tribes selling mercenaries to the Roman armies, but because they are substantially different from the Etruscan/Roman forms they are sometimes connected with alternative forms of Old Italic alphabets. On a related note, despite the fact that the Romans occupied much of the British isles from 43 to 410 AD the Welsh language was also not written with the Latin alphabet until afterwards, the earliest examples found being from the 6th c. AD.
It was not until the arrival of Christian missionaries who came to Anglo-Saxon England from the late 6th c that English began to be written systematically with the Latin alphabet. The most famous mission was sent by Pope Gregory from Rome. Others came from Frisia and Ireland. This was part of a drive to improve the ability of the English to use Latin in order to spread Roman-Catholic Christianity through means such as the training of Priests. It was also designed to make it easier for Rome to keep tabs on the English.
The 23 letters were adapted to the sounds of Old English and four new symbols were added to indicate sounds found in old English, but not Latin.
'Ash' æ (occasionally still seen in British English, e.g 'Encyclopædia' but considered archaic)
'Thorn' Þ which comes from the Runic alphabet and is still found in modern Icelandic. This usually represented a voiceless 'th' sound such as in 'Thor' 'Thusday' or 'They'
Eth ' ð' represented a voiced 'th' sound as in 'the' or 'their', which later came to be written as Y, hence 'Ye Gods really says 'the Gods' and Wynn 'Ƿ' which represented the sound of our modern W.
(If you're not sure about voiced versus unvoiced practice saying 'sip' and 'zip' or 'fox' and vox' in English the voiced form of S is Z and the voiced form of F is V. Try to notice how your vocal cords vibrate for voiced sounds, they feel as though they are being produced in the throat or further back in the mouth.)
Another letter 'yogh' ʒ was introduced from Ireland in the 8th c, this was used to represent the 'ch' sound found in Scots 'loch' and the 'dg' sound found in 'bridge'
When the Normans arrived in the 11th c these Anglo-Saxon letters began to disappear. Ash was replaced with a, thorn and eth by th, yogh by gh (Night would have been pronounced like Scots 'Nicht' in Old English) and wynn with w.
However these changes happened very gradually, thorn persisted in to early modern English and the advent of Printing and ash and yogh are still seen today. Ash in words like encyclopaedia as mentioned above and yogh written with the z symbol in Scots English, so that the name Menzies is pronounced as' Men-ghis'
In late middle English the letters J and V were introduced as variants of i and u. Originally these were simply alternative ways of writing these letters but gradually they became associated with the sounds of loan words from French and Scandinavian languages such as 'justice' and Viking'
And so...
We have our fully formed, ready to use, modern English alphabet with 26 letters in place by late Middle English (the period of the War of the Roses) but variations persist in widespread use in to the 18th c. and in regional and specialist use until today. That's the gist of it, but I've oversimplified LOADS so check my sources below or consult /r/linguistics if you want to go in to more depth. A good way to simplify it if you want to study it for yourself is to look at the individual letters and their histories rather than the alphabet as a whole.
Sources and resources.
Inception to Greek
https://www.inkling.com/read/history-of-graphic-design-philip-meggs-5th/chapter-2/the-north-semitic-alphabet[3] http://en.wikipedia.org/wiki/Proto-Sinaitic_script[4] http://en.wikipedia.org/wiki/Cuneiform_script[5]
Greek to Latin
http://en.wikipedia.org/wiki/Old_Italic_script#Raetic_alphabets[6] http://en.wikipedia.org/wiki/Latin_alphabet[7] http://en.wikipedia.org/wiki/Greek_alphabet[8]
English alphabet
The Cambridge Enclyopaedia of the English language by David Crystal The story of yogh from BBC H2g2[9] http://en.wikipedia.org/wiki/Old_English_Latin_alphabet[10]
Latin influence on English -student essay with good citations[11]
http://en.wikipedia.org/wiki/Anglo-Saxon_settlement_of_Britain[12] http://en.wikipedia.org/wiki/Middle_English#Late_Middle_English[13] http://www.omniglot.com/writing/oldenglish.htm[14] http://en.wikipedia.org/wiki/Gregorian_mission[15] http://en.wikipedia.org/wiki/Yogh#Old_English[16] http://en.wikipedia.org/wiki/Wynn[17] http://en.wikipedia.org/wiki/Eth[18] http://en.wikipedia.org/wiki/Thorn_(letter)[19] http://en.wikipedia.org/wiki/%C3%86#English[20] Related alphabets http://en.wikipedia.org/wiki/Welsh_orthography#History[21] http://en.wikipedia.org/wiki/Runes[22] http://en.wikipedia.org/wiki/Anglo-Saxon_runes
Note that the order of the English alphabet has multiple sources. The A symbol (or variants thereof) has been in the initial position since the Phoenicians, although the sound it represents has altered, whereas J is a newcomer, put next to i as it began as a variant of i. Each individual letter has it's own story, there are common narratives which connect many of them, such as the age-old practice of learning by rote which made an order that was easy to recite more memorable, but there's no overall explanation for the order, it's just a series of events through thousands of years which gave us what we have today.
This is a surprisingly common question around here! You may be interested in the FAQ section about this.