Hello!
I am sorry I totally disagree with the "solution" of setting up Mac address filtering proposed by another member.
It does not help you, it will makes you think you are secure while even a kid can override it!
To reset your password you need to "reset" the router, either by pressing the Reset button on the router (probably on the back of it, with a pen as its a small button). Please note that sometimes you have to wait for like 15 secondes before it reset it! Also try at last solution, you need to press and leave that button pressed, unplugged completely from power, replug, still pressing...
Otherwise do you know you can access the "router config page" with typing like 192.168.1.1 or 192.168.0.1 or 198.168.2.1 in your browser? (firefox, internet explorer...)
The thing is you seems to get confused, reseting the password will not help you preventing someone else using your connection via wireless..
to be secure you will need to:
-change the default login/password of your router config
-put encryption Wpa2 for the wifi
-put a proper complex passphrase (not like myteddy123, something hard and long like "z#~@'2g@rd3n!!"T3stPou.?".
-lower the broadcasting range settings if you don't it to emit the signals very far
-change the ip addressing of your network (ie change from 192.168.1.1 by default probably to something hard like 10.134.21.1)
-check regularly the logs if available in your router config page (192.168.1.1)
Well if you do that thats already in good way!