This is the most important part of a bike. It has to be strong enough to absorb impact from the trail, yet light enough to enable the rider to carry speed and climb hills. Aluminum is a popular frame type with these characteristics, though you will also find steel. Ask somebody who works at the bike shop to help you find a bike that fits your measurements. Basically, you should be able to stand over the top tube with at least an inch of clearance between it and your crotch and be able to comfortably reach the handlebars while having a slightly arched back in the riding position.
Mountain bikes have shocks that take the place of the front fork.
You can t go wrong with shocks from major manufacturers such as Rock Shox, Marzocchi or Manitou. Some bike manufacturers, such as Cannondale, make their own shocks. Most XC bikers prefer clipless pedals. Clipless is a misnomer, because the pedals actually allow the rider to place a cleat attached to the sole of the shoe into a spring-loaded clip. This is known as clipping in.