Assuming the wall is not load bearing, you could screw through the ceiling. Assuming you're tiling the shower, check out www.johnbridge.com for help on tiling. Also, look into Kerdi system from www.schluter.com. It's neat way to build a shower (I put one in for my mother), and allows you to use drywall for your walls since the membrane is totally waterproof. It's nice to have the waterproof layer immediately beneath the tile - the shower dries out much faster, and is a great system.