Wingnut already gave you some excellent pointers/advice, which I am not going to reiterate.

It seems you haven't enrolled in or committed to any FFP (frequent flyer program). When you decide whichever one to be your "commitment", don't forget to check in the FT airline subforum and see if there's a current referral bonus promo. That will easily net you some extra miles/points.

Another thing I want to point out is most of the airliners do not give retrospective miles if the passengers fail to enroll in their FFPs BEFORE the flights or WHILE in the flights. You didn't mention which airline you flew for your taken LHR-SFO trip. So you might want to check into the terms for that specific airline. For example, if it's UA, you are out of luck.

You may feel "overwhelmed" being a FT newbie. If you find time, check out the stickies on top of each forum, especially in the airline forum of your interest. The stickies are very informative and will help you get used to the FT "lingo" in a short period of time.

Welcome to FT!