Bitcoin-qt doesn't currently support the creation nor the redemption of multisignature addresses. Creating multisignature addresses is pretty straightforward with bitcoind using the "createmultisig" command. You just specify m and n, and a list of public keys, and it gives you a multisig address. The difficult part is sending a payment from that address at a later time. Bitcoin-qt doesn't allow you to specify the address you send coins from; you have to create a raw transaction in order to do that.
The commands you'll have to use are:
createmultisig
createrawtransaction
signrawtransaction
decoderawtransaction
sendrawtransaction
Gavin Andresen's
TwoOfThree.sh is an example script that shows you how to create a multisig address and send coins from it, step by step. That would probably be the best place to start.
Warning: Be careful when using the raw transaction API. You can easily create invalid transactions or transactions that will never be confirmed, resulting in the destruction of your coins.
Edit: blockchain.info's web wallet allows easy creation and redemption of multisig addresses in a graphical fashion. Although not the most secure way, that would probably be the easiest way of creating a multisig address.