If you like someone and you tell them then this a very good thing because you are being up front and honest. If they send you mixed messages then it sounds to me like they are somewhat interested but not completely certain about it.
I noticed that you are in high school. You are still very young and people that are usually in this age group are uncertain about becoming involved with someone. I think that this is completely normal behavior so I would not worry about it too much.
For now the best thing to do is just be cool about it. If you are really interested in this person then you certainly do not want to be too overpowering as this may scare them away. If you two really click then in time you will find that it is going in the right direction.
You are learning a valuable bit of information here...relationships stink! Okay, that might be a bit dramatic, but the same forces that make love wonderful are the same ones that make it a real pain in the keester.
There are lots of possible explanations for your beau's behavior, so I'll just pick a couple out to blow hot air about:
1. At your age (and I mean that with respect...not in a"talking down to" kind of way) young men and women are just starting to get an idea of what they want out of a "significant other." It may be that he is still trying to figure that out and isn't sure if you fit the bill or not.
2. He probably has little experience with real relationships and is still figuring out how to act...he will learn this by watching the behaviors of the people he cares about. For example, if he does "like you" and he isn't acting the right way, you're counter-behaviors will give him a clue that he needs to act differently or risk losing you.
3. He may be a jerk and might be amusing himself with your affections.
4. Because you care about him, you probably have a heightened sensitivity to what he says/does/doesn't do. You, too, are also learning about the subtle clues of how a relationship works. It may be that you are misreading his cues because you care enough to try to figure them out.
5. Boys, traditionally, mature more slowly than girls. If it makes you feel any better, he might be just as confused about your relationship (or the possibility of a relationship) as you are.
6. Boys that age are generally morons. Men my age are generally morons. In fact, guys are pretty dumb when it comes to relationships all around.
From my perspective, the important thing is the be honest and assertive about what you want from a relationship while still maintaining your dignity. If you don't understand his behavior, ask him directly. That might seem a bit forward, but if you do it in a "non-clingy, non-psycho, non-stalker" way there is no better way to get the answers you are looking for.
Whatever you do, remember to be yourself. Be confident and don't cheapen your worth by chasing someone around who "runs hot and cold." Boys like girls that present an obtainable challenge and who take pride in themselves. Girls who are too compromising and eager to please (though it runs contrary to logic) do not earn respect and are not valued (other than by weirdo, control freaks.)
Good luck to you. You are at an exciting age, and in some ways, I really envy you. Every door is open and everything is still a possibility. Don't sell yourself short.
The best way to let someone know that you like them is to be a good friend. Listen to his or her problems, be there when you are needed, and just enjoy each others company. At your age, relationships will come and go - you have a long life ahead of you and the people you are friends with today will go on to college and to lives that may cause you to drift apart, but true friendships last a lifetime. I still have friends from high school, many of whom I have reconnected with as a result of the Internet. We have all gone in different directions in life, but the people I have remained closest to have been those who I counted as friends. So, if you really like this person, my advice to you is to be a friend. Don't pressure yourself or this other person to be anything more defined than a friend. If something grows out of it as you move on in life, then it was meant to be something more. If it does not, however, you will have a friend and that is something that is very special!
Now, as to things that you can also do to let a friend know that he or she is important to you - cards and small presents are always good. Even a friendly e-mail just to let someone know that you are thinking about him or her can brighten that person's day immeasurably!
Best of luck!