I have a bunch of different ideas for books or stories, how do I decide which one to write first?

All writers have a lot of ideas. Nobody is in the situation where they get only one idea at a time and the second one pops up usefully when you’re done writing the first one. Keep a notebook, write your ideas down. Eventually, however, you’re going to have to commit to one. Here’s a useful article on deciding if your idea is novel-worthy.