How do you dump a boy who is to clingy and you don't know why you actually said yes when he asked you?

You used the term "boy" so I assume both of you are fairly young. It takes practice to know how to ask someone out on a date, how to act on a date and sometimes some boys are shy and don't communicate well and don't keep in touch, while others can be clingy. The best thing to do is communicate with him and don't hurt his feelings. Just say you aren't ready for a relationship right now and it's best to part company. If he says he wants to remain friends with you then tell him that you would prefer not as it only complicates the situation. Be careful, some guys can be stalkers and not even know it themselves. When you break this friendship up then do it in a crowded cafe or somewhere where your school friends are nearby.