To pick a board you need to consider your goals. Do you want to travel with it? Run rivers? Race? Surf? Just have fun? What is your fitness level and how is your balance?
Gleich recommends trying a bunch of boards before buying. Many shops carry rental or demo boards. If you live in the mountains, check with your local whitewater kayak shop or try a Google search. Another option is visiting supconnect.com to see if there are free demos, anyone in your area to paddle with, or local events. Ask your SUP friends if you can try their boards. The most important rule for starting is to find a board you love.
For sizing, a 10-foot board is good for most people—the wider and thicker a board, the more stable. If you want to enter races or paddle long distances on flat water, consider a longer, 12-foot board, which will be the most efficient. For surfing, you’ll want to downsize, so you can turn easier on waves. For running rivers, look for a board that can handle unforeseen impacts, like hitting rocks. For portability and travel, inflatable boards are the best option, although you will sacrifice performance on flat water and surf.