How can I password my dlink to stop people from use it with out my permission

We'll need more information.
What sort of D-link ?
Wired, wireless or thought connections ?
Domained or workgrouped computers?

If this is a home wireless router, you need to review the manual on setting security. It varies from brand to brand and even from model to model. Generally, you log into a configuration page on the router by using the router's IP address. From there you set the security the way you want.

Type the IP address of your D-Link device in a browser and then enter using a admin name and password, if its not changed then by default it's admin and admin for both.

Go under Wireless --> Wireless Security-->and password encryption.
I suggest WPA as WEP can be cracked.