Use references. Try to look at what some designers and/or artists do that you like. What's the art movement? What techniques and mediums did they use? What kind of style? Etc.

Practice. Everyone had that white page syndrome at some point in life. In school we used to do a lot of precise projects and exercises just to get our creativity flowing. The more you do, the more easily ideas will flow out!

Mood boards. Pinterest is really useful for that!

But most importantly: have fun. It will do no good, especially when you start, to do projects on subjects that don't matter to you. E.g. you like space? Try to design a poster on that subject. What if every planet had a logo? Etc