Steps to choose seat for UA:
1. register for Mileage Plus online at https://www.ua2go.com/ci/JoinMileagePlus.jsp?navSource=Dropdown07&linkTitle=join
after this you will get your account, which is a 11 digits number.(When registering, sometimes using a Chinese Address is not ok, so just use your American Address. Or just an random one, as we can change it in the future. It is the same with Phone Number, just enter 0 for all is ok).
2. Call 800-810-8282, to ask the officer to add your flights to your account.
3.Refresh UA website(or login UA website), your flights will be there
4.Click “Seat assignment”, then select the seat you want.
PS: There is a website telling us which seat is better, here is the link http://www.seatguru.com/airlines/United_Airlines/United_Airlines_Boeing_747-400.php