Hi, if you already have a working DSL line and an account from any ISP, you can simply enter these details into your modem then you will have WiFi.
Do you have a modem and does it have WiFi? If not you should buy a router to broadcast your internet connection from your modem or buy a modem that has WiFi.
Here is a general guide how to set up a modem with your DSL details: http://www.betterbuilt.com/docs/ppoe_setup_07252005.pdf
If this doesn't help, can you give more details on your modem, thanks.
Hope this answered your question, if it did, please mark as the correct answer for the next viewer. Thanks! :)