If you head over to TartyBikes and click on the 'Tartybikes' Guide to: Bikes' link (located in the little grey box) there's a nice overview of what size bikes suit different riders. You'll find the majority of new riders purchase an Onza due to the low prices (as an unexperienced rider you'll be bashing the bike about a lot etc) and upgrade to a bike with a better specification once their riding ability has progressed. It's worthwhile posting your budget, as well - that'll give us a better idea of what to recommend! Oh, and bare in mind that BMX and trials riding are quite different! Well, unless you're Macaskill...