In regards to what he needs to supply for my visa....is it his birth certificate, passport copy, accomodation, bank statements, paystubs?
Just to let you know that if he is willing to send his original passport, he doesn't have to send his birth certificate - they need either a) original passport, b) certified copy of passport or c) photocopy of passport plus the long form of his birth certificate.
His bank statements must be up to date and must also be officially signed/stamped by the bank. If you are not applying very soon after he gives you the bank statements, he may need to send you more recent ones in the post (depending on when you apply in the US).
When i visit in two weeks that will be my first time in the UK, will customs hassle me? Ive read horror stories online! And I am totally scared!
Customs won't do anything to you unless you are bringing in fresh meat, fruit/veg produce, animals or other banned substances
. Customs have nothing to do with letting you into the country - they just deal with what you may have brought with you in your suitcase after you have already been let in and have gone through baggage claim. It is immigration that will determine whether or not you are allowed into the UK to start with (and are the ones who can give you hassle) - if they don't let you in, then you won't even make it to customs at all (a lot of people use customs and immigration interchangeably, when they are actually two completely different departments).
In terms of whether or not you are hassled by immigration, you need to have enough proof as you can that you are a genuine visitor and plan to leave the UK at the end of your trip. A definite must is a return ticket. You also need to show that you have enough money to cover all your expenses for the duration of your visit and ties to the US to show that you have a reason to return - such as evidence of US homeownership or a rental agreement, a letter from your job stating when you will be returning to work, a letter from your school (if you are a student) stating that you are enrolled and will be attending classes, evidence of family living in the US that you will be returning to etc. How long are you visiting the UK for? If it is a short trip, then you probably won't have too much to worry about, but if you were trying to stay for several weeks/months, then the immigration officer (IO) is more likely to question your motives.
Most importantly: be honest! If they ask you who you're visiting don't just say 'a friend' if he is actually your boyfriend/fiance - be honest and say that you are visiting your boyfriend/fiance. If you say 'friend' at first and then they question you further and find out he is more than a friend it will be considered deception and you can get into trouble for lying to an IO! However, don't give more information than they ask for.... just give short, concise, truthful answers (i.e. What is your purpose of visit? I'm visiting my boyfriend).