~Your local retailer is an expert~If they know their snowboards ~
1.Type - Figure out what type of riding your going to be doing the most.
Eg. Freeride, Freestyle, Freecarve or Race
2.Length - If you're going to be riding pipe, you'll want a shorter board about a couple cm below your chin so you can spin & do tricks easier. If your going to be riding big-mountain, you'll want a longer board from your noise to the top of your head so you can go over powder & be more stable at high speeds.
3.Flex - You can check the flex of a board to see if it's to soft or to stiff. If you're a beginner, you'll want a softer board.Also if you are ride halfpipe or doing trick riding. So you do not snap or damage the board If you're a advanced rider, you'll want a stiff board.Mostly freecarve or Racing
4.One other thing, don't choose a board by graphics. Sometimes if you choose a board by its graphics it quality is sometime crap!. Also if you have sweet graphics you always want to keep the board clean.So you will not try to pull of sweet moves
Any more questions about selecting on board email-me