Good helpful thread.
To answer a lot of basic questions, it would be helpful if we could point to threads answering:
I bought a United ticket, but it's on USAirways, or vice versa.
Where do I check in?
Can I check-in online? Through which website?
Can I use the Kiosks?
Who do I call to make changes? What about seat assignments?
What if the first leg is United, then others are on USAirways?
(Please feel free to add others)
I've been a US and United elite for the past year and I'm still confused by some of these.