Extract private keys from Bitcoin Wallet
Use pywallet to extract the private keys and copy the output to blockchain.info's Import Backup page.

To use pywallet, you open a command prompt/terminal window at the folder where you saved it and type ./pywallet --dumpwallet --password="Whatever your passphrase is". NOTE: this will output everything in plain text, so if you do this on a public computer or with somebody looking over your shoulder, some of your bitcoins may be stolen.