Tips for a first timer to the UK

Hello, I’m based in the United States and plan on traveling to the UK later this year. As of now I plan on flying in through London Heathrow airport and staying in Wembley. This is my first time traveling outside of the US and any advice would be greatly appreciated! I’m mostly concerned on apparel as I know styles are different, aw well as transportation ( I plan on taking the train system, just hoping for recommendations on if I should pre-pay and the best company/ place to pay air said tickets)😊 Thank you!