Trish has given great info and definitely stuff you should follow. I just wanted to add a couple personal comments:
I had about 30 years driving experience in the US before I moved across. I drove both manuals and automatics and didn't seem to have a problem switching over to the other side of the road. Even with that I can't stress the importance of having had professional instruction. I was able to find a teacher who would tell me how to pass the test; not just how to drive. Armed with this info, I basically soared through the tests (one small glitch, but otherwise fine).
Since there are so many manual cars here, it might be best to learn how to drive a manual, and take the test for that so you're just going through the process once. It's not impossible to get an automatic here...just limiting if you can only drive an automatic.
I was told that the US officials consider it illegal to have two driving licenses, even if they are from two different countries. In other words, once I passed my UK test, I should have destroyed my US one. I'm just saying what I was told by my ex who is a cop...I don't agree with it so therefore I didn't follow the advice. (Why the heck should I do what my ex says?)