SKIP THIS IF YOUR STEAM GUARD IS ALREADY ENABLED
Step1: Open your Steam Client
Step 2: Click on the Steam button located on the menu at the top left of the client. Once you have the menu open click on Settings.
Step 3: Click on the ‘Verify Email Address…’ button.
Step 4: Follow the prompts and click on the link provided by Steam to the e-mail address to finalize the verification process.
Once the address is verified your settings page will look similar to the screenshot below. Notice how it says Steam Guard is not enabled.
Step 5: Restart the Steam Client.
Step 6: Return to the Settings page the shield icon will now be green and it will provide you with a date you turned the service on.
Step 7: You will be able to trade 15 days after activating your steam guard.