I have a hard time saying no?

I'm a junior, and I'm starting to realize that I have a big problem when it comes to guys. I have a really hard time saying no to them, and it's starting to affect my reputation. Whenever a guy and I are "talking" , I just put all my effort into pleasing them and making them happy and trying not to lose them. Now I haven't slept with the whole school, it's only been about three guys. One of them a guy I dated for three weeks, one a guy with a girlfriend who told me they broke up and he needed me, and the other just a senior player. But the latest guy really hurt me, and I've been hearing rumors about myself. Now this self-proclaimed "nice guy" and I have been getting to know each other. And now he's talking about "doing stuff" and I find myself trying to think of ways to say no without hurting him, when I really should just stick up for myself. Is there any way for me to get a backbone and just get stronger before this wrecks my entire high school reputation?