You should always give your guy the opportunity to realize for himself that you like him so that he can do something about it if he wants to. Be a little flirty and be sure to spend time with him. Try to introduce little touches and other hints. Just don't drag this on too long! Try biting your lip every time he looks at you or smile nervously. Look into his eyes and then turn away slowly. It's important to choose a good time to talk. You don't want him to be distracted, upset about something else, or just downright busy! That will kill your chances before you even get started. Talk to him about setting aside some good time to talk, or try to catch him at a time when you know he won't be busy. Talk to him in private. Confessing your crush in front of other people will make him feel pressured and embarrassed, and you don't want that! People that feel cornered can be unwilling to admit how they really feel. Instead, get him on his own so that the two of you can have a real heart-to-heart. When confessing your feelings, you have to just get out there and say how you feel. Be confident! Guys will think this is sexy. You also have to be bold because you're going to need to take the initiative in talking about how you feel and whether the two of you could have anything.

TL;DR in Vietnamese:
Gửi cho chàng vài tín hiệu. Tìm một thời điểm thích hợp. Trò chuyện khi chàng đang ở một mình. Hãy can đảm.