The most important thing is to make friends with the staff at your local bike shop – they’ll help you out with the sizing and fit, may also know someone who’s selling the right bike for you.
Try a few shops, stick with the one where the staff seem friendliest and keen to help.
Also, have a look in the back of “What Mountain Bike” magazine – they recommend a couple of new bikes at about £350 that seem tough enough to last a while.
If you’ve got any mates who ride regularly, or can find someone local who knows what they’re talking about, secondhand is a really good idea, but can be expensive and even dangerous if you get it wrong. There really is some tat out there.
It’ll be a hardtail at that price, unless you get a real bargain, but a lot of people think that they’re more fun anyway!
Good luck, let us know how you get on.