I'm starting the game next week. Any advice?

I get my new computer set up next week. I'm really excited to play this game. I've wanted to for years now but never had a good computer to play it on.
Do any of you have advice for me? Mistakes to avoid? Things to do right away? Things you wish you had done differently? I've never played computer games before, always Nintendo and Sony so this is a new experience to me all together. I'm worried about getting used to the controls.