do a search on this site for tiling a tub. There has been a number of threads explaining a couple different ways to deal with the tub lip. As far as layout and starting, the top of a tub is seldom perfectly straight. Most tilers start one row up from the bottom, leaving just under a full tile for the first row so the tiles can be trimmed to fit. Use a small piece of wood (1 x1, 1 x 2, etc.) and fasten it to the wall with a couple of screws. The wood strip will support your tile when starting with the second row. You can remove the strip after the thinset has firmed up enough to keep the tiles from sagging. Caulk the few screw holes with silicone seal and cut and fit your first row. Leave them about an 1/8" off the top of the tub and caulk that joint when your done grouting. Also caulk any 90 degree corners. You can get caulk that matches the grout color at the tile store.