You like him, but the feeling isn't mutual. It sucks and you'd give anything to change the way he sees you. Unfortunately, we can't force feelings where there aren't any. We can, however, work against fanning the flames of a debilitating crush.
Here are some quick tips on how to pick yourself up and dust yourself off after experiencing rejection:
Express Your Feelings

Some people might shrug and say, "So he turned you down? Get over it." If it was that easy, wouldn't you have already gotten over it? Rejection is tough. It takes time to get through. One of the first steps to getting through it is to accept that it happened and that it made you feel especially crappy. So, express how it made you feel. If you're not into talking to a friend or parent about it, then write your feelings in a journal. Bad feelings are like snot, they feel gross when they're taking up space inside of you and they may even feel gross/embarrassing as they come out, but they've got to come out. So, "blow your nose" by expressing yourself to a trustworthy friend or to the pages of a journal.
Reassess Your Strengths & Positive Qualities

When a crush isn't reciprocated you might find yourself sort of obsessing over "what's wrong" with you and trying to figure out why he doesn't like you. That's a dangerous road to travel. Instead, take time to remind yourself of your value. Do this by making an actual list of things you admire about yourself. You can write the list (if you're a list person like I am) or you can craft a list in dream board style. This means you'd use pictures and drawings to symbolize various aspects of your personality. This is the perfect time to remind yourself of what makes you uniquely you.
Be Determined To Make New Friends

Keep your current friends close, but now is a great time to expand your friendships. Just a little while ago, you were ready to pull someone new into your inner circle. But he missed his opportunity and now that leaves room for you to fill his space with someone new! It doesn't have to be a romantic relationship, it can a new friendship. Join a club or class that allows you to meet new people who share your interests and be determined to get to know them!
So, this is the opposite of a fun time in your life, but you're going to be okay. Be a friend to yourself by allowing yourself to cry and feel whatever you need to feel. Once you've cried, get a firm grasp on who you are and what makes you uniquely you, then be determined to make new friends. Doing this will be a big help as you maneuver your way through the tricky terrain of a broken heart.