Going to USA in a few weeks, anything I need to know?

In about a fortnight I plan to go to USA for a holiday with friends. We plan to be there for around 3 weeks to a month and I was wondering with a stay that long if there was anything I needed such as a visa? I'm a British citizen and my pport was renewed only about 2 months ago. I saw something about an electronic visitor p and was wondering if I had to apply for one? Also, I was in America about 2 years ago for a cl trip, does this change anything?