Look for a pad that can be folded in half for easy carrying. Good ones often include sewn-on handles and padded shoulder straps that allow you to schlepp the pad like a backpack and use it to carry gear like shoes and a chalk bag. Make sure the pad is durable and can hold up to repeated falls and abrasive rocks. The inside should consist of layered foam and the outer shell should be rip-proof. Buy a pad that makes sense for you.
Pads come in many sizes, widths, weights, and shapes.
Which one is best depends on where you like to boulder, how high you tend to get off the ground and other personal traits. Wait to spend money on a crash pad until you're sure bouldering is your sport. Old mattresses, couch cushions and gym mats all work well in a pinch. In that case, take your shopping to the Internet. Bouldering is a physically demanding sport that could result in serious injury.