Does anybody know any good sites or book that help you learn to improv and develope your own style? I feel like all I can do is play what I read, but ultimately I want to perform and write my own music. Or any tip on how to develope as a more jazz or rock style cellist?

The best advise I can give you is for you to go to a college or a school that teaches jazz. It depends on how much theory you know. You need to have a quite advanced knowledge of theory of music to actually start writing music and having it sound good. The internet cannot teach you as much as a teacher who spent many years getting their degree in composition. I would try to actually get into a class to learn how. That will guarantee you will be able to write your own music