First is immigration, than you pick up your baggage and go to customs. Your baggage at ICN should be tagged to IAD .... if it is, you can then drop it at the recheck desk after customs. If it is not tagged to go to IAD but only to LAX, you may have to carry it to UA checkin at T7, so try to get it properly tagged at ICN check-in. T7 is a 10 minute walk from TBIT; there is also a free bus that takes you between terminals, but the walk is quite nice after a long flight. As for BP, you can normally get that at ICN.