Both yet and already are used with the present perfect tense. Already is usually used in positive sentences. Yet is usually used in questions and negative sentences.
Imagine that you and your friend are going to travel. There are many things to do, and you ask your friend if he has done these things: Have you bought the tickets yet? Have you arranged a taxi yet? Have you reserved the hotel room yet? Have you packed the bags yet?
In all the examples, use yet at the end of the question. Your friend might answer: Yes, I've already bought the tickets. Yes, I've already arranged a taxi. No, I haven't reserved the room yet. No, I haven't packed the bags yet.
Use already in the positive answers, and yet in the negative answers.
There is one time you can use already in questions: it's when something happens earlier than expected. If your son finishes his homework in just 15 minutes, you could ask: "Have you already finished your homework?!" because you were expecting it to take more time.