I'd suggest dumpster diving for old aquariums. Dis-assemple them and recycle the glass. This will give you free glass to learn on. Unlike wood you can not "trim" small cuts from glass. At 3/8" thick you need to be making at least 2" cuts.
Buy a good quality glass cutter, leather gloves, eye protection and lots of sand paper & emery cloth. Wear eye protection, Glass shards are an excellent way to destroy your eyes.
Don't buy glass until you've made a few tanks. Professionally cut & polished edge glass will cost as much or more than a pre made tank.