I guess you want to know the difference of these two words in the Perfect tense. In some languages the translation of "already" and "yet" is different, so just by translating the sentence you know which of these two words you want to use. But there are also some languages, in which they are translated the same way. In that case, you need to know the following rule:
Already is used in declarative sentences. (+)
- I have already done my homework.
Yet is used in interrogative (?) and negative (-) sentences.
- Have you done your homework yet?
- I haven't done my homework yet.
Hope this helps you out!