Enabling wi-fi is different for each mobile device. Look into the manual or the Settings menu or connections menu, etc..
First of all, check your wireless modem security. This security should also be compatible with your mobile device. WPA or WPA2 or WEP. Determine the SSID of your modem.
When you want to make the connection after enabling the wi-fi capability of your mobile device, choose the correct SSID and click connect. Then you will need to enter the password. Make sure remember the password is checked in the mobile device.