A first journey to London - pehaps a first journey to another continent - is a huge step and you're right to be wondering lots of things. I'm born and bred in Englnd, and I remember my first intercontinental trip (to Ottawa, Canada of all places). Very exciting, very new and yet ... another city, civilised, full of people who are 98% like the people at home; of course I worried before I went there but it turned out to be a lovely place with lovely people. I'm sure you'll find your experience in England the same.
Now - let me see - what did you ask
For the journey Your passport (you a US citizen? if so, you can come for a holiday without a Visa) and your return ticket (in case you're asked to show that your really are a visitor at immigration). Check out the "Visas and Citizenship" board. As a genuine visitor, you'll sail in, but try to "sneak in" for other reasons and the folks at passport control are trained to spot you!
Bring your personal effects and what you'll need for while here - customs are much more concerned about illegal substances, firearms and professional smuggling that they are about your day to day stuff. But remember there's no point in bringing things you can buy locally, which is just about anything. Pack anything that's sharp in your checked luggage - not for customs, but for the airline security folks.
For when you're here Warm clothes (you're from LA? It will be 10 to 15 degrees colder most of the time) and comfortable shoes (I expect you'll be using the excellent public transportation for most journeys).
Beyond that - hey - in many ways the UK is the "52nd state" with just that 2% difference. You'll find a McDonald's on every corner in London (my daughter works for them!), and many of the other chains too. You'll find "Friends" on the TV. But there are enough differences to make a trip here a real pleasure - have a look through some of the posts to other boards on this forum - you'll find many questions already answered, and people far better qualified than "your travel correspondent" to answer questions on refrigerated meat.
There's a lot to do in London. Take the tube (Subway) and the bus - epsecially try route 11 which takes you past many of the main tourist sites in company of the locals. Ride the London Eye, see the Tower of London, Westmister Abbey, the river, Greenwich. Take a trip to the Theatre to see live shows - there must be around 30 shows every evening. And sample the food - try a pub lunch (and an unchilled beer), have "fish and chips".
Many good trips out of London too, but I suspect I've alredy said enough to get you started ... do post any follow ups and I and many others will chip in with their ideas and suggestions based on the sort of things you like to do.
