Things I would consider.....
--Volume: 2 bottle (~20 oz) will probably get you through a lot of training. I use 2, and the only time I felt I needed more was doing long runs of more than 10-15 miles in hot conidtions.
--storage: what do you want to carry with you? Fuel for longer runs, identification. If you want a cell phone or other things, you should make sure you get a belt with enough storage or loops (if you do gels)
--Fit/Form: Besides betlt, there are hand-held and packs. You may want to look at those. As for fit, go try them on at a running store. You need to see which one will work for you.
For example, I ended up with a Nathan Speed belt 2 bottle, because I liked how it set on my back, I liked that the bottle cages were rigid-so I could just reach around and grab it and easily put it back without stopping. The one I have has a small pouch, which is sufficient for ID, 4 gels but not really anything more. When I started to train for marathon during the summer, I bought a Camelback Annadel so I could get more volume. It has more storage, but getting access to the gels on the run is more difficult.
Good luck finding one that works. I would suggest looking at Nathan, Amphipod, Fuel Belt, Camelbak. They are pretty commonly found and you can get an idea of what you like.
Maura