I just went through a similar situation.
My husband proposed to me this June, on my birthday. We were going to apply for a fiance visa, have me move over, and then be married within 6 months. However, in August we received a newsletter from the UK Immigration site that I subscribed to, stating that the visa eligibility age is going to be changed from 18 to 21 effective this December. Well, we're both 19, so that was a huge problem. There was no way we would be able to get a fiance visa, plan a wedding, marry, and get a spouse visa by December; and that's just time wise. The cost was out of the question! He had been visiting until July. So he had to fly back here and we had a backyard wedding. I thought I would be able to go back with him, but they needed his passport and a lot of other paperwork that he needed to be in the UK to get. Things actually just finally sorted themselves out yesterday- I got my visa in the mail today.
Everyone is right, you don't need anything special to marry in the US. All we did was apply for the typical marriage license and we were ready to run.
I'm glad that you have a year to plan this, because it has been the most stressful time in my life. I even started having sress related reactions where I would develop swellings on different parts of my body! It will be a much easier process for you, as opposed to only having a few months. The thing to remember is you have to have evidence to back up and prove every little thing you claim. There's a lot of paperwork involved. I didn't find this site until after I'd submitted my visa, but definitely stick around. These people are amazing, and you will have a much easier go at it all. I know it's confusing, but relax. It'll be fine and it'll be worth it. I wish you luck!