I'm not American, so I took the UK test as a teenager driving for the first time, but
1) It took both my brother and I three attempts to pass, but our younger brother passed 1st time (I think only about 50% of people, maybe less, pass first time - it's known for being a hard test to pass and there are rumours of daily pass/fail quota for the examiners although the DVLA deny it)
2) It depends on the test centre, but at mine it was about a 2-month wait: my 1st test was booked for early April, but I failed and the next available date was early June. Failed again, and the earliest date after that was 1st August. However, the examiner was ill and it got postponed so I finally passed on August 9th, 15 months after my first driving lesson (it took me ages to gain enough confidence to take the test)!
3) As the others have said, if you wish to drive a manual in the future you must take the test in a manual, if not, you can take it in an automatic but you will not be licenced to drive a manual.
4) Even if you've been driving in the US for years, it's wise to take some lessons because there are certain things you have to do on the UK test in order to pass that you might not be familiar with or you might normally do slightly differently.