I have a question about checking in, and picking seats.

I am flying to Korea via Singapore Air. I have a connecting flight with US Airways to SFO, from which, I fly to Korea from there.

I got my travel reservations through a travel agency, got my reservation number, ticket number, etc.

Question I have is, I put in the request to get window seats. The travel agency says I am required to check in and from there the seats are assigned. But after talking to a friend, I was told that I should be able to pick my seats ahead of time before check in. If I call up Singapore, should I be able to do this?

Also, since I have a connecting flight. Am I able to do Internet Check In for my Singapore flight ahead of time before I even get to the Airport? I am not sure if this will conflict with anything (as I will be going to the airport, and checking in my bags with US Airways?). Not sure how this all works, or if I have to check in at the airport since I have to do a connecting flight, like if they have to do both check ins at the same time to make sure my baggage gets to the correct destination, etc.

Mainly, I want to be sure I have window seats, and if I have to check in (if I am unable to get seats ahead of time), I was going to check in via internet as soon as I am able, but not sure if that will mess up anything with the US Airway flight check in and with my baggage and all?

edit: I checked via Singapore Airlines website my Booking, but it seems that I am unable to change seats. It is as if I have not even been assigned a seat yet as it doesn't show what seat I have. I am thinking that changing seats via their website via booking is only available if you bought tickets via their website (and I did mine through a travel agency)