You can book anywhere you like. Some websites will allow you to enter your United Mileage Plus number into a US Air reservation, some won't. If you book on a website that won't, you can "apply" your United MP# by either calling US Airways or doing it when you check-in at the airport.

United and US Airways are both part of the Star Alliance, so you can use either program for a flight with either airline. You can also use your miles in one program to get a free flight on either airline (or any other member of the Star Alliance). However, you cannot combine miles from two programs to get one flight. Therefore, I highly recommend that you use your United MP# for all your United and US Airways flights from now on.

Good luck!