While the internet can be a great tool for researching something new, the amount of information can also be a bit over whelming. So, I'm going to break this down to a simple form - then if you want to continue researching - give you a link to continue your quest for more knowledge.
First answer these questions:
1. Where are you paddling? Flatwater or Surf?
2. How much do you want to spend? A trend, the more you spend, the lighter the board and easier to haul.
3. What type of paddling do you plan on doing? Here is a quick categorization of boards based on type of paddling courtesy of EMS:
Surf: shorter boards that turn well and are naturally at home in the waves
Family Recreation: durable boards with width for stability
Cruise: long boards, often with room for cargo; at home on flat water
Fitness and Race: long, narrow boards built for speed in any water conditions
Yoga: wide, stable boards; often made with full deck pads for better grip in various postures
Ok, now if you are like me, this is where you stop reading, go find a board and jump on the water. There is a huge selection of boards out there. If you are new to SUP, and I'm assuming you are as advanced SUPers know exactly what they need and want, then my advice is to get a moderately priced board that fits the shape you are looking for - something you can cruise around on and experiment with and aren't afraid to bang up a little. In my opinion, too much research stalls the goal… and that is to get off the computer, on the water, and start living!
"Life has no limit - if you're not afraid to get in it" - Mason Jennings
Now, if you are still feeling the need for more knowledge - EMS has a very informational article about the entire make-up of the board and how it may affect your choice: Read it here.
Also here is a list of a few board manufacturers if you need a place to start comparing boards. Please note box stores like Menards and Walmart also carry boards if your choice is largely based on price point. Both ways are commendable - Getting out there is the important part!