In the Swedish navy, this varies quite a bit during the 18th century. First of all, the Swedish navy was manned mostly by indelta sailors, which meant that the men had a croft or a fishoping house to support them and their family while they served in the fleet.

From the 1680s, a man could not avance beyond NCO ranks without having attended the navigation school at Karlskrona (this school branched out to Stockholm and Göreborg during the latter half of the 18th century).

The Swedish fleet laid up during winter, when much of the Baltic Sea was frozen over and navigation was impossible.

An officer could either start as a sailor and advance up to NCO and then become an officer by studying at the navigational school - which seem to have been surprisingly common. Only about 25% of the officers were of noble extraction 1687. The other option was to be sent to the navigational school as a youngster and join the fleet as an officer at young age.

Promotion was done on seniority and competence, except for the absoltue highest ranks, which were appointed on political grounds. For example, Admiral Duke Karl, nominally the commander at the Battle of Hogland 1788 (the brother of the King) was appointed because of his position and support of the King, and spent most of the battle drinking heavily and trying to bribe the commander of the flagship to bring her out of the firing line. Real command was held by Otto Henrik Nordenskiöld.

Becoming a Captain in the Royal Navy was based on both merit and influence. The first steps in becoming a captain of a man-o-war began for most around the age of 11-13. A captain would take a young hopeful aboard so that he could learn his trade. So you come aboard the ship as a first-class volunteer and after a few years you become a midshipman. The youthful start was needed because to take the Lieutenant Examination you'd need 6 years at sea, certificates of competence and good behavior from the captain(s), journals, and "technically" be aged 20.

Passing the oral examination on seamanship and navigation would earn you a commission as lieutenant. Here influence would often help as there were not always positions open for LTs. If you weren't aboard a ship you'd be on shore earning half pay. Let's assume you'd be posted to a good ship with a good captain. Your next step is to be promoted to Master and Commander(later just Commander). This promotion is not based on an examination; only by distinguishing yourself can you earn further advancement.

Okay, you've served for a few years and moved from 3rd to 1st Lieutenant and your ship fought a winning action. The Admiralty decides to promote you the M&C. The only ship you can now command is a sloop-of-war. Similar to the move from LT to M&C, you now must make a name for yourself to become a Post-Captain. Post-Captains can command anything from a 20 gun post-ships & frigates to 100 gun First-Rates. Post-Captains are "posted" to a list and promotion to admiral is based solely on seniority. When one captain is promoted or dies, you move up the list. Unless you disgrace yourself, you'll eventually become an rear-admiral. Becoming an admiral does exactly mean you'll be commanding a fleet. If your career wasn't the greatest you can be "yellowed." Fleets of the age were set up in colors; blue, white, and red. An admiral of the yellow achieved the rank but has not be given a fleet to command.

"Six Frigates" by Ian Toll gives a great look at the lives and careers of American and British naval officers in the late 18th/early 19th centuries.

http://www.amazon.com/Six-Frigates-Epic-History-Founding/dp/039333032X

Very readable, and there's also a lot of fascinating stuff about the construction, provisioning, and arming of warships, and of course detailed accounts of battles.