Hey man i am 19 and have just got into riding too, i have bought a downhill bike myself which i love to bits. Im very new to MTB too as i have always ridden a BMX. There are plenty of web sites you can check out to choose your bike, thats how i started off. Another wicked way of buying a bike is looking in the best MTB bike magazines available to you, they may have lots of offers for reduced bikes. I found mine on e-bay with £100 off as it was coming to the end of the season, it was brand new and from a legitimate bike store. Hope this is some help to you, good luck bike hunting!