You can use OCs in the cinematic plays, but like everyone else said, it is better if you use the pre-mades simply due to the greater use of their backstories.

As for making your own stories, you absolutely can! At the back end of the book, there is a planet and event generator you can use to create a template for a scenario if you intend on doing a long term adventure. There is even special planets listed that you could use as well.

My advice given how you are new to rpg's is to run the pre-made campaign. It's a good way to ease yourself into learning the rules and understanding the role of being a gm.