Buy the best bike you can afford. Get the right size bike. Until recently, most mountain bikes have been designed for a 180 pound male rider. Look for a smaller bike and try out several models. This distance should be a minimum of two inches.
Proper fit is important to avoid injuries and to provide maximum maneuverability for a safe ride.
Buy a helmet which meets either the ANSI or Snell certifications. Look for a sticker, both inside the helmet and on the box, to make sure your helmet meets the safety standards set by these organizations.