Goods are generally the same as using a CC! Go to your site of choice checkout and pay with BTC! I've bought a few things (mostly computer components) with BTC!
Exchanges on the other hand, are a little trickier. You will have to set up an account with MtGox.com, or TradeHill.com, or another exchange. There you will click on an "add funds" button, it'll give you a BitCoin address to send BTC to. Send them from the client/wallet.
Then to cash out it's a bit trickier yet. You will have to register an account at a "middle man" online bank such as: Dwolla.com, LibertyReserve.com or the like. Then after registration, you will need to link your bank account to them. I'd suggest opening up an account at your physical bank primarily for this, so you aren't disclosing your personal account info to the online banks!
So, to conclude; To get BTC from your wallet to USD in your hand, it would look like this:
Wallet.dat->MtGox.com->Dwolla.com->your bank account->your hand! =)
The whole process once everything is set up is ~2days to cash out!
There are other ways, (check by mail, Cash by mail, privately using PP/Amazon payment, Ruxxum...etc)