You only have two options:
1. If you have no knowledge of computer/networking, then call your service provider and they will come to your house to reset the password.
2. If you have knowledge of computers and networks and know the URL and admin password of your WIFI router, then you need to connect your PC/laptop to the router directly using network cable.The cable might be provided to you by the service provider. Once the connection is done, in the browser type the URL of the router, this will open a page and ask for admin password (if you don't know this password, ask from your service provider, it is generally set to a default value). Once on the webpage, explore the site and various tabs eg "Networking", "Setup" etc. There will be an option somewhere to set the password for WIFI connection, you can reset your password there. It may be possible to see the old password also.