What are bitcoins?
Bitcoin is a decentralised virtual currency, meaning neither does it exist in the physical world.
some exchangers are:
mtgox.com/‎
www.exante.eu
www.wm-center.com/
preev.com/‎
The above inf, is gathered from google. 
I cannot afford to buy bitcoins. so i have started earning with my current balance 0 bitcoins.