You need to do a lot of research. It's important to find a gallery that is a good fit for your work. Many galleries have submission guidelines on their websites, and often prefer to look at your work via CD. If they are interested in seeing prints, then bringing in a portfolio of unmounted prints is fine. Yes, generally, 50%. They are usually responsible for the marketing costs (and their own overhead, rent, staffing, etc.), and you are responsible for making the pictures, and increasingly, for the matting and framing. It's best to set your prices for prints only, and they take the 50% from the sales of prints. At the moment, I have one gallery representing me, and if someone wants a print, I will make it when they order it. It's a good investment to have a show "ready to hang"... in other words.. 20 or so prints framed and ready to go. These can be your exhibition prints that aren't necessarily for sale (artist proofs if you edition) and fill orders as they happen, or have the extra prints in your or the gallery's flat files.