Assuming the router is still under factory settings the username/password combination will be one of two things;

On older routers;

USERNAME:
PASSWORD: admin

On newer routers;

USERNAME: admin
PASSWORD: admin

If you previously changed the username/password and have now forgotten them (or someone else changed them because you hadn't enable security encryption) you'll need to reset the router first by holding down the reset button for 30 seconds. Then use the values above.

Once you are into the configuration screen you can change this username and password.

Just changing the username and password wont stop people hijacking your wireless. To prevent this you need to enable WEP or WPA encryption, which requires you to set a passkey.