Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
You have to login to your router to secure it. On your computer that is connected to the network (wired/ethernet connection is preferred), open your internet browser (Safari) and goto this website ---> http://192.168.1.1
This will open your router configuration window. When asked for username and password, enter admin as password ***** leave username blank. Click OK (This is the default username and password ***** access router configuration. If that doesn't work, then you have to reset the router and setup it again. To reset press the reset button on the back of the router for 5-10 seconds and this will reset. Then login to your router using the default username and password. Remember resetting router will temporarily disconnect internet until you setup it again from the router configuration window.)In the Configuration page, click on Wireless tab. Then in there, just under Wireless tab, you can see the Wireless Security tab. Enable security there.
If you have any questions or would like any additional information, please don't hesitate to ask.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.