1) In the UK we enjoy moaning, we enjoy complaining about the weather (even if it's nice) and are not as openly loud and proud as Americans are, don't get me wrong we love our country as much as Americans love America but people don't tend to say things like 'I'm proud to be British' very often (cringe).
2) I know I may be bias but you will bloody love London, the only really comparable city in America is New York (cities like Los Angeles cover a massive area and are not really proper industrial cities) but it is such a great place, I don't live there, I live in a place called Dorset in the South-West which is really nice but I love a trip to London, sooooo much to do.
3) However try and get a taste for UK culture while you are here and don't stick to just London, Make trips to places like Stonehenge, Cotswolds and other cities such as Birmingham, Manchester and Edinburgh, they are all really great cities.
Also try and get a taste for British culture, I know you are a girl but do things like go to a football match in London (you will never experience a greater joy than a good atmosphere at a football match) I know loads of girls that get really into the football and you will be very popular if you know your stuff, just find a few other people and go and watch someone like Crystal Palace or West Ham. Try out British food, people have a bad view of British food but actually I think it is really nice,not only things like Fish and chips and roast beef (but a do love them) but things like Stew and Dumplings (OMFG IT"S SO NICE) or shepherds pie are really nice, if you don't like it then there is food from every singe country in the world in London (including plenty of American things)
4) Generally anything you can do In America you can do in the UK so there is really not much to worry about, Crime in London really isn't that bad. There are cultural differences but you will pick them up quickly.
5) Don't worry about looking out of place in London there are sooooo many tourists and people from all over the world that people expect it so don't be surprised.
6) AVOID ALL UKIP VOTERS