First off, I'd invest in a pair of Loeka Digger shorts ($64; loeka.com). These lightweight microsuede polyester shorts, designed by an upstart company out of Canada, are the most comfortable mountain bike shorts I own at the moment. That could have something to do with the stretch panel in the rear, which helps them move with you when you're climbing or riding technical singletrack. One thing: They have no lining because they're designed to fit over a chamois, but they're still surprisingly comfortable without it.
As for bikes, you need to go to a good shop where the salespeople can ask you all sorts of good questions, like how you ride and with whom you ride. But promise me this: Don't start out too low. Mountain biking, perhaps more than any other sport, requires good gear from the get-go. You need a tough, durable bike that you can confidently take from forest service road to singletrack. There's nothing that will dampen a ride more than a low-quality bike. Trek has been working hard on its WSD women-specific line. You might want to check out the Fuel X8 ($2,300; trekbikes.com). This full-suspension, aluminum, X-C bike is super lightweight, has a Fox 32 F-Series RL shock, Shimano SLX/XT components, and 120mm of travel. If funds aren't an issue, I dream about Trek's Top Fuel 9.8 WSD ($4720). With 100mm of travel, a carbon frame, Fox 32 F-Series RL shock, and Shimano XT components, this bike is virtually indestructible.