Several ways.
Buy them at an exchange like Mt.Gox.
Mine for them (lokup info in the mining subforum).
Get them from sites that give them away for free like
CoinChat, or
BitVisitor.
If you're into gambling I think
BitVegas (a Bitcoin Casino created in Minecraft) is nice too.
However your starting point will be to create a so called wallet, so you are able to receive BitCoins. For Howtos either lookup these forums or Bitcoin.org.