beau d.
Assistant Pro
Get yourself down to a proper bike shop and test and sample the bikes in the shop,usually the lads working in them will look after you and take you through anything you need to know...
If you go for say MTB each make will have something slightly different so for instance you might think Giant and Specialized look the same but if you sit or even better get to ride them the riding positions will be slightly different and one may feel more comfortable than the other so my advice is get to proper bike dealer..
I agree with what you say hamshanker, however if you are on a budget and don't mind getting your hands dirty Halfords do offer some good budget bikes. If you're serious about biking then main dealer is the best place....