You need to enter the configuration page and turn on WPA2 security and enter a passphrase that will be needed by any connecting devices. Your manual and THIS guide gives you some details of how to find that in your router.
You may also want to turn on MAC address security if your router supports it and feel the added security is needed. If possible turn off WPS, which can be cracked by Reaver to bypass your WPA2 security on routers that use WPS.
One last idea is to change your admin ID and password for your router and write them down somewhere that you won't lose them.
Look at manual for specific instructions, but it is in the configuration pages of the router under Wireless Security.
To get to the configuration page you have to type in the router IP address, for DLink usually 192.168.0.1 (or type in http://dlinkrouter) into the browser of an attached computer, usual password Admin or admin, and password blank (or admin I think on some old models), then go into Wireless Settings (wizard or manual) and look for wireless security mode -- set it to WPA or WPA2, and a passphrase.
There should be a whole section in your manual called wireless security that has pictures of the configuration pages.
Some of the newer DLink models have a Wireless Security Setup Wizard that will tell you step by step what to enter -- network name (SSID), security type, and a passphrase. Write the passphrase down as all of your devices will need it too.