What can I expect babysitting a 3 year old?

I'm babysitting 2 kids I have never met before next week, one is a 3 year old boy. I really don't have a lot of experience babysitting such a young kid other than my sister. I do know that he is potty trained, but will he need a reminder to go to the bathroom so he doesn't wet himself? Will he need naps? I have to watch him all day, any advice for me? I'm really nervous.