travelling 2015 usa americaWatch
I've spent time in New Orleans and Baltimore alone, and was generally able to make temporary friends by just striking up conversations in bars, coffee shops, book stores, etc. "Hi, I'm English, what's good around here?" works remarkably well.
If you're going on a gap year I'm guessing you're under 21? That might cause you some issues, as some places won't let you get a hotel room or rent a car under 21. But with a little advance planning you can work around that.
You can also looking at sites like Couchsurfing. Be careful to avoid the weirdos obviously, but people on there are generally friendly. It's a cheaper alternative to hotels and generally provides a supply of locals to talk to and learn about cool local stuff from.
How long are you thinking of being in the states for? Any particular area? Do you want to stay in one area, or get a car and drive around, or take the Amtrak/Greyhound/domestic flights between cities?
The reason is that there's barely any public transport in the US, except in the major cities, but to get from city to city you need a car. The problem with this is that all the car rental companies add on a massive fee to compensate for drivers under 21. If you take an 'full adult' (21+ years old), you might avoid this problem altogether though.
It's definitely a cool place to go, I suggest you pop into Canada for some brilliant scenery. But most of all, head into Mexico if possible, it's a fantastic country and there isn't much of a language barrier at all despite it being hispanic.
I second what other posters have said, go on a tour if you go to the US. Or wait it out until you're 21 and you can really get the most of the USA then. You might want to consider Europe instead by the way, it's very friendly and safe for young people, plus they offer huge discounts on under 21s. The transport and food is amazing, and there's loads to do.