US Airways is in Terminal 1 Concouse A
United Domestic in Terminal 3 Concourse F
United International in the International Terminal Concourse G

Codeshare flights take place in their respective carrier's terminal. If UA codeshare on US metal then T1 and US codeshare on UA metal then in T3 or International depending on destination. Check-in on codeshares will take place at the airline operating the flight's counters.

US and UA check-in are not within easy walking distance with a lot of luggage. It's best to take the AirTrain to the respective terminals. The outer AirTrain, closest to the terminals, runs counterclockwise so T1-T2-T3. The AirTrain, closest to the parking lot, runs clockwise so T3-T2-T1. It should only take about 5 minutes to go from US to UA.

If you want to walk, the best route is to go through the International Terminal which connects to each of them at respective ends through moving walkways. If you are in no hurry, it can be a nice walk as the International Terminal is quite new and impressive. It puts the other Terminals to shame.

Make sure to check your tickets to see if codeshare or not--very important in determining your check-in counters.

UA Map of SFO http://www.united.com/page/article/0,,1121,00.html
SFO Website http://www.flysfo.com/default.asp