The sooner you start your search the better!
If you are planning to apply to a US university, there are things you can start early (in fact you are probably doing a lot of them already). There are really three key reasons to do this:
1. Universities are looking for consistent and improving academic performance over time
2. They are looking for sustained activities and community involvement that show commitment and depth of engagement with your personal interests and the wider world
3. There is a lot to learn about the numerous US institutions and you might need some time to get to grips with it all!
Term 1, Year 10
Your focus in year 11 really has to be your GCSEs - I can't overstate this. Since the disappearance of the AS Level, your GCSEs will be the ONLY official indicator of your academic abilities by which universities can assess you*
Term 1, Year 11
and ACT (http://www.crackact.com/) tests.
Contact colleges you are interested in, ask them questions, explore their websites!
Familiarise yourself with the commonapp - https://www.commonapp.org/
* apart from the SAT and ACT (which are not a test of academic excellence, or intelligence necessarily) - for info see resources page
If you have just decided to apply to the USA, don't worry - there is plenty of time and you have probably done more than you think already. However, this is when the process starts in earnest. Of course your A Levels will still be your academic priority - they are universities' greatest indicator of your academic abilities, and they are MUCH harder than GCSEs, so give them your all.
But, you should be keeping up with / building on your community involvement and personal projects and you do have some other stuff to do...
Term 1, Year 12
Things will accelerate quickly in Year 13 and it will be important to balance your academics with the requirements of the application process. You can apply to unlimited US colleges, but each will cost you in both money and time. If you have done your research you should be able to make a selection of around 6 that cover all the bases.
Term 1, Year 13
Term 2, Year 13