Algae buildup on the walls of my tank...How do you get rid of it?

I have a 10 gal. gl tank and I keep getting algae in it like crazy and I don't know why! I never had this prob. before in plastic tanks. I have one goldfish in my 10 gal. which shouldn't do much to the tank. Does anyone know how to stop algae from forming on my tank walls??? HELP!