If you're just starting then whatever you do don't buy a full suspension bike. Get a semi-decent hardtail and improve your skill and technique while your confidence grows and you become more knowledgable. Then depending on what type of trails you have readily available to you, buy a better bike as you progress.
There is so much choice for bikes nowadays, I reccommend going to bike shops and having a browse and chatting to shop assistants for help on which one to get. Browse bike websites for a better idea of what starter bikes are the best.
http://www.bikeradar.com/
http://dirt.mpora.com/
Starter hardtails range in price, usually between £300-800.
Make sure it has hydraulic disk brakes, decent drivetrain, good forks etc cause you don't want some shitty thing that will break after every run.