As geeta said, you can drive for 12 months on your US licence, but you still have to pass the UK driving tests if you want to continue driving after that. If you haven't passed the tests when the 12 months is up, then you will be treated like a learner driver (no motorways, always accompanied by a UK driver, L plates on the car etc.).
So, your first step will be applying for a provisional licence (which you need in order to take the UK tests).
Then you can study for the theory test, and take it, and after that, you can take the practical test.
It's probably a good idea to take some driving lessons before taking the practical test, so you can 'learn how to pass the test', as they are very specific about what you need to do to pass and most test centres only have about a 45% first-time pass rate.
I would recommend getting started on the UK licence process as soon as you can, as it can potentially take a few months to prepare for and pass all the tests (especially as some test centres can have long waiting times for tests).