So the theme has been announced and they said go make games, but it says the jam doesn't start for a few more hours. I'm confused as to when we start. Also what are the rules about assets? Do they have to be self made or can you use other sources?

Use any assets, just not over the top. You can start if you want :) Giving yourself a time limit is more satisfying in the end though some people have started b/c obligations over the next two days.