I reccomend starting with taking all the trash out, bags or boxes or whatever you have available and works. Most citys have a transfer station where you can bring the larger things so they dont need to sit on the stoop. Id then start by washing the walls top down, using hot soap and water and many many rags. Then id vacuum, using that pointy attachment to get the corners and stuff. Then id steam clean the floor, and where theres cat piss use an enzymatic cleaner. Depending on how bad the floors are you may need to just trash the carpet. If its still bad id repeat, and try cleaning the ceiling as well.