If you have any friends that play, borrow a few discs and find out what works best for you. Each disc is rated specifically for the power it requires to fly properly, how it will glide, fade, and turn. Simply put, they are all different.
Otherwise, check out the manufacturer's websites for better ideas on how each disc will work, and you can select a disc that seems appropriate and then purchase online. It also helps to get an understanding of the plastic types, as they do change the feel and flight of the discs, but most importantly the price.
The 2 main disc manufacturers: