What you should ask depends. You can keep your questions open-ended if you don't know them very well yet. Ask them what they did over the weekend. What do they do for fun? You can compliment them on something (their shoes, Gameboy, a recent accomplishment, new haircut, etc.). That should lead to some better questions, "Where did you get them?", "What are your favorite games?" Try to listen to their answer and follow-up with some related questions. If you tap into a topic of interest, you'll notice them answering in more detail. Ultimately, if you like him, you'll want to lead up to questions that will end up with you spending more time with him. If you are at the same school, ask him where he goes for lunch. Ask him if he's seen a movie yet that you would like to see. Ask him if he's going to the baseball/basketball/football game or concert. You can also ask if he's got a blog or Facebook/MySpace account. Then get his online handle so you can chat online.
Try and find something they like. Let them talk about it for a while and then if you notice something else they say in a conversation ask about that. Start with basic stuff. "What classes are you taking?" "Whose your teacher" "Do you like sports" "How long have you been playing the." Then find stuff you can relate too. normally the conversations will start to flow on afterwards and you can go from the notes you've picked up of the things he's said.