There are a number of threads that deal with this, mamadou... basically, your phone has WiFi already installed, but you have to disconnect the costly Cingular GPRS connection that automatically tries to connect every time you use your browser...
You can test the WiFi connection at any Starbucks... you'll get an invitation to pay for the WiFi... but you will know that it's working!!!
I remember we had a thread about this last week, also for a 8125, but the title had to do with disconnecting the GPRS...
Sorry I don't have time to search it out for you.... :-/
Good luck, and let us know how it goes!