When searching for the right surface to create your artwork on, you have a few options. I prefer to go to my local Blick Art Supply and purchase "Blick Studio Artists' Boards. I use the "Flat" style, take a look.
Its a heavy duty panel, that is primed with gesso (which is awesome and helpful), it won't warp, they're reasonably priced, they come in standard framing sizes and unique sizes, and has built in convenient hang holes on the back.
The gesso allows you to draw or paint on the panel before you glue down your glass. And also allows the adhered glass to grip the panel
You can visit a local store or obviously shop online. If you are trying to save a few bucks, you can go to home depot and purchase ply wood. They'll most likely cut it for you but I wouldn't trust them if you know how to use a saw. I went there for my very first piece and the cut was not straight. Wood also warps over time and change of climate. - joshy b