I have a trip to korea, and I am flying 2 different airlines, on the same itinerary

LAS > SFO US Airways
SFO > ICN Singapore

If I wanted to do airline check-in, who do I check in with? Do I need to checkin with both airlines? or does checking in with one, check me in for the 2nd flight?

Also, if I have bags to check-in, do I just go to the airport and check in bags afterwards?

SQ let you do online check-in 48 hours before flight, so just try there first (at 48 hours before your SFO-ICN flight) to see if you can check-in for the SFO-ICN flight. Then at 24 hours before your LAS-SFO flight, do online checkin for the USAirways one.

You will still have to go to USAirways' counter at LAS to drop off your bags anyways.

The nice thing about doing online check-in ASAP is that you can often switch to better seats that were blocked prior.

You can check your bags all the way through to ICN at US Air in LAS. Just show them your SQ flight details and they'll transfer your bags at SFO for you.

I just read that I am unable to checkin with Singapore because my preceding flight isn't with singapore. Checked around, maybe it is because I have to checkin with US Airways. so looks like I will have to try 24 hours (if 48 hours doesnt work).

You will probably have to check in at SFO for your Singapore flight.

I had to do the same traveling on US Air to LAX and then had to check in at Thai Airlines even though my luggage was checked all the way through to BKK. The desk personnel wanted to see my passport and luggage receipts before they would issue me my boarding pass.