Along with other security best practices, the best way you can secure your wallet is to encrypt it - follow these steps:
- While you have the wallet open, go to Settings > Encrypt wallet
- Enter a strong password and either remember, or keep this somewhere securely
- Restart the wallet for the changes to take effect
An encrypted wallet will restrict certain actions. To do things like sending coins or starting masternodes, you will need to unlock the wallet. Navigate to Settings > Unlock wallet and enter the password you used to first encrypt it.
Note that staking will not be possible without unlocking the wallet, however you can enable staking this way but more securely by limiting the functions allowed in the wallet through Settings > Unlock wallet and selecting the "For staking only" option.