How does one society create new words with a new alphabet and find meaning for these words? And how was the first language created?
Please note that I'm keeping this super simple, to the point that I'm glossing over details and perhaps over-simplifying some points. If you want clarification on anything, just ask.
To start, languages are not generally created in this way. They aren't created out of whole cloth. There are of course "conlangs", languages like Klingon or Dothraki or Esperanto which are created, but these are rare and not something linguists really deal with since they often lack the full depth and complexity of any natural language.
Your question is really multiple questions which are best addressed separately, since the only common thread is that they're topics in linguistics. So, if I were to reword your question, it'd be:
How do new words arise in existing languages?
How do alphabets develop?
How did the first language evolve?
Very important to these re-phrasings is dropping the word "create", since languages develop largely independent from the will of the speakers.
How do new words arise in existing languages? See the last character for how the arise in the first place in the earliest languages. However in languages with speakers, the most common ways that new words arise is by people coining new terms (often things like company names or the kind of jargon used in specialist fields that may have not previously had that meaning), through combining existing words ("tele" + "phone", though that's more like a coining. also "black hole", which itself isn't that old of a word.), or through sound changes over time. The kind of words that are intentionally created are called neologisms.
How do alphabets develop?
A large number of the worlds writing systems originally came about as pictographic systems. For example, the letter A is likely ultimately derived from a picture of a cow's head, which has been stylised and now no longer really looks like a cow's head. The pictograph represented a word which would have started with an A sound, and thus it developed to just be a phonetic representation, losing the connection to cows. The Chinese character 月 was originally a pictograph of the moon, which has now become stylised to not look like the moon any more. Chinese writing and English writing derive from very similar earlier systems that simply developed in slightly different directions. For modern new words, it's just a matter of fitting the symbols that represent sounds to match the sounds people make when saying a new word (e.g. I say "floin" out loud and you could easily come up with letters that could represent that word on paper).
How did the first language evolve?
We're not sure. For many, this is the ultimate question in linguistics. It is not currently known if all languages share a common ancestor, or if language developed independently in multiple places around the world. There are conflicting theories about how language or the first language came about, but it's all just educated guesswork at this point. We simply don't have the answer to this question.
You should try to post your question to /r/AskAnthropology , I think you might find some help there!