In order to stake your coins from the QT Wallet you must first encrypt your wallet. You cannot stake from a wallet that is not encrypted.
Simply select "Unlock For Staking..." from the Settings menu and enter in your wallet passphrase.
Staking with the UI Wallet is quite easy as well. Simply select "Staking" from the left-hand menu of the wallet and toggle the staking slider. You will then be prompted to enter in your passphrase.
You can enact finer control over staking by using Wallet staking options in your nix.conf configuration file.