Get to know each different bike, see what you like the best.. see what appeals to you.. what parts you would like on a bike..
What I would recommend is..
An Atomik RS, Atomik Nitrous 138cc, MSO x6r, MSO x7r..etc They are some of the good bikes you can get for about 800-900.. Look around more though..
You can get second hand thumpstars and chinese bikes, but i wouldnt advise it. Knowing the quality of these bikes [Not very good] It will probably fall apart on your first ride.. Even if the seller says there is nothing wrong with it.. You never know.. ive been conned by a dodgy dealer and it ended up costing me about 600 in repairs, 750 for the bike and in the end the engine needed a rebuild so i just sold it..
Not worth wasting your money a second hand chinese bike, or a total piece of crap [Demon, Lei, MXB [In Exception for daniels bike.. because he has modded it to the shithouse.], Foxico..]