How long have you been in the UK? Your US licence is only valid to drive in the UK for the first 12 months after you move here. So, if it has been longer than 12 months, you will have to start from scratch with getting a UK licence, as if you were a learner driver.
Most cars are manual, though you can buy automatics more easily now than a few years ago. If you take your UK test in an automatic, you will only be licensed to drive automatic cars, which means no manuals, ever. However, if you take your test in a manual, you will be licensed to drive both manual and automatic cars.
The first step is to apply for a provisional driving licence (this will involve mailing your passport with the application if you are not a UK citizen).
Once you have the provisional licence, you can start practicing your driving. However, as a 'learner' driver, you will be restricted to learner rules... which means:
- you must display red 'L' plates on the car at all times
- you must be accompanied at all times by someone over 21 who has held a full UK licence for at least 3 years
- you will not be allowed to drive on motorways
It's a good idea to take a few driving lessons first, so you can learn how to a) drive a manual and b) pass the test (it's very specific and only about 50% of people pass on the first try). You can use your instructor's car for the lessons and to take the test in (most UK teens learning to drive won't have their own car yet, so this is common practice).
You will need to take and pass the theory and hazard perception tests first, then the practical driving test.
Once you have done all of that (which may take several months - it took me about 1.5 years of lessons, plus 3 tests, to get my driving licence when I was 17), you will have a full UK license and will be able to drive. However, insurance is likely to be expensive as you will be treated as a 'new' driver. I'm not sure whether you would have any issues with renting cars as a new driver - you may be considered an 'at risk' driver for the first couple of years.