I don't know how to say no
Why do you think you can't say no?
I don't want to disappoint anyone.
Although you might not want to disappoint anyone, sometimes it is in your best interest to say no. If you never refuse others they will walk all over you.
I get pushed around by everyone anyway.
Probably that's the reason you're getting pushed around. People take you for granted if you say yes to everything they say. It's alright to say no.