What should I use to fill in the screw holes from the shower door ?
the shower curtain will be more colorful and work fine === covering those holes that are left from the removal of the door should be discussed with the paint center at either Lowe's or the HoDepot as they may be able to near match the fiberglass walls with a caulk or silicone and then therew is no painting === all shower curtains come in a standard width and length six  feet -- trim to fit the opening == Walmart has a good tension rod [[so duz HoDepot]] makes it easy to fit into place and you can put the curtain and rings on rod while not in mid-air ... If you live near an IKEA one of their good buys does happen to be the liner and decorative curtains .. check it out ...
It is a shower stall from the 1980's. It is some kind of plastic.
If it has ceramic tile, just squirt some white GE RTV silicone sealant into each old hole and smooth it off with a wet finger.
Silicone caulk should work very well. A small tube of it will be enough to fill in a few screw holes.
Hole Plugs http://www.homedepot.com/s/hole+plugs?NCNI-5 Scroll down they have many kinds
we had to do the same thing when we moved in to our home that is what it had we just had a pro do it