Write what comes into your head and go from there. Even if you think it's crap, do not rewrite anything because often times your first ideas are your best.
Secondly, watch a lot of movies, especially animated ones. This will help you develop your comic well as comics are very similar to film. Pay attention to detail, dialogue, and characters.
Thirdly, have a flashdrive and/or bookshelf dedicated to inspiration. Find things that inspire you, and fill it up. That way you can have something you can reference.
Lastly, daydream a lot. This will help you get to know your characters and be immersed in their worlds. When you know your characters inside out, when they feel like they are real, then you're all set.
Have fun, and good luck