Most People can ride a bike, so have the general skills to start Mountain Biking. The best way to improve on these skills is to join a Mountain Biking club, have a look in your area, with the sport becoming so popular there are Mountain Biking clubs popping up all over the country. Riding with like minded individuals is fun and conducive to improvement.
How much do I spend on a Mountain Bike$%: At first, not too much, if it's not the sport for you then spending a fortune will be a waste. As your skills improve you will naturally want a better Mountain Bike, I suggest a 'HardTail' at first, which has front suspension forks and a rigid rear end. Look to spend no less than :%$pound;450, this will buy a Mountain Bike that will not fall apart after a few rides.
When investing in a bike make sure you try out many different brands, and if you can, test ride these. Most Mountain Bike shops will have a 'loaner' that you can use, or ask a friend to let you ride his bike. When settled on your brand and style of bike, why not get the previous years model, most bike shops will have these at a heavily discounted price sometimes as much as 50% off.
As your skills improve you may want to invest in a Full Suspension Bike which has front suspension forks and a rear suspension shock. This will depend on which discipline of Mountain Biking you prefer, these disciplines can be categorized as follows:-
XC - cross country - as the name suggests riding or racing cross country off road, probably the most popular category. Suits either front suspension or full suspension lightweight bike.
Downhill - riding on very fast downhill sections - suits a full suspension Mountain Bike which needs to be very strong, also a full face Helmet (motorbike style) and body amour should be worn.
Free-ride - riding on man made obstacles such as ladder drops, north shore, skinnys, this discipline is usually extreme, body armor, full face helmet and a full suspension very strong Mountain Bike are required.
Dirt Jumping - jumping off man made dirt jumps which are sometimes 8 foot high. Suits usually a very strong Hardtail
4x - 4 riders compete together down a course made up of jumps, banked corners (berms) and humps. Suits either Hardtail or Full suspension bikes.
Whatever discipline you prefer, you will always need a helmet, it may not feel 'cool' but then neither is a fractured skull. I know which one I would choose!! Protective glasses are also a good idea, with knobbly tires mud and stones will get flicked up into your face, so it makes sense to protect your eyes. Gloves are also a good investment and will save many a 'skinned' knuckle.
Flat and clipless types of pedals are used in Mountain Biking, flat pedals usually lean towards dirt jumping so you can 'bail out' if the jump goes wrong. clipless pedals allow the rider to lock into the pedals mechanism for extra power and speed, especially when climbing a steep hill on sometimes very technical terrain.