Bring the tile down to about 1/8" of the tub surface. Caulk all changes of plane (i.e., the corners) and changes from one material to another (i.e., at the tub edge to the tile). It sometimes helps to fill the tub with some water prior to caulking in case it settles. Around the windowsill, maybe 8-10" all around it, buy something like RedGuard (available at Home Depot and other places where it may be cheaper) and paint it on per the manufacturer's instructions. This stuff is a waterproofing paint. Moderately expensive as paint goes, but it is cheap insurance. Once it dries, then you can put the tile up over it and, with the RedGuard under it, you can tile the sill if you want. It is red and kind of thick, you can brush, roll, or trowel it on. Since it will be covered up, it doesn't matter much what it looks like as long as you cover things well. BTW, I learned this reading stuff at www.johnbridge.com and I'm not a pro. Depending on the tile, make your grout line appropriate (some tile are not very consistent in size from one to the other and it is hard to hide this if you use a small grout line). For grout lines >1/8", use unsanded grout, for 1/8" or greater, sanded is usually used, otherwise, if you use unsanded, since it shrinks some when it cures, you'll end up with cracking. Once everything is cured out, depending on the manufacturer but usually a week or so, put a good penetrating sealer on the grout.