If your crush has no idea you exist, it's going to be difficult to attract him. You need to find ways to attract him even before you start talking directly to him. Try to get to know each other by becoming acquainted with his friends, joining a club he's in, or asking someone who knows him to introduce you. Be careful because if you don't want him to know you like him, they might say something which could ruin your chances and make things uncomfortable.  If your crush is in your class, talk to him a little before or after class (talking during class could get you some unwanted, negative attention). You could comment on the teacher, the work you have to do or make a joke when he's listening. It's scary but you'll feel good once you make a move. Say something like, “The homework was hard last night. Were you able to figure it out?” to start a conversation with him. It's easier to start conversations when you both have found something to talk about. You might possibly consider taking up some of his interests but don't become a whole different person just for someone else. It's definitely not worth it. If you have no common interests, don't freak out, just ask him about his hobbies or about something you already know he enjoys. For example, if you've never watched a TV show before that he's practically addicted to, just tell him, ask him about it to spark an instant conversation. You could say, “Have you ever seen [TV show you know he likes]?” to get him talking about something he is a fan of and will get excited about. You have to be friends before he will start to reciprocate the whole "crush feeling" in return. If you aren’t already friends, you could always start out by bringing up a general conversation topic (i.e. weather, a crazy thing that happened that day, presidential elections, assignments...) and if all else fails, you could always just ask for a pencil or some paper and go from there.  Maybe after you've gotten to know him a little better, you can joke around or tease him, maybe even flirt with him...

TL;DR in Vietnamese:
Thân thiện. Tìm điểm chung. Tìm hiểu thêm về anh ấy. Vui vẻ.