I agree with Tanya about finding a good accountant. The most important thing to do is make sure there is a clear track of everything. So make sure you take the time to set up clear simple accounting from the get go. In terms of an accountant, you should see if you can find one that is familiar with global accounting, or at the very least accounting in both India and the US, as they will need to be familiar with both tax systems in order for you or your company not to have problems with either governments, and also for you not to be double taxed.
To simplify things in terms of customers in the UK I would suggest either having them continue pay directly to India or move to US Business account when you are set up (the sooner the US is set up and money is coming in, it may also benefit your immigration status depending on how you are entering the US). Introducing a third currency and account will further complicate things.
Definitely get that accountant sooner that later as it may be a lot more complicated than just figuring out which form is needed. It's worth it to just get set up properly on all fronts from the get go. It usually isn't that expensive to get them to help you set up and then you can manage it on your own.
Best of Luck!