First, click in help > debug window > console. In this console you must use the dumpprivkey command in the following form
 
dumpprivkey yourzoinaddresshere
 
After introducing this command, you will receive your private key. This private key should be written down and saved.
 
If your wallet is encrypted, you must use first the command walletpassphrase first.
 
walletpassphrase yourpassphrasehere 60
 
Where, yourpassphrase is the passphrase you used when encrypted your wallet and timeout is the time you want to unencrypt your wallet for (in seconds, eg. 60)