WiFi Router IP Problem Found

Initially I was using a DSL Modem, now I needed a WiFi system. I purchased a WiFi Router for enabling WiFi system in my premises. I installed the drivers and connected the DSL Modem and WiFi Router with the Computer System. After completing all this, I found an error, "IP error".
I want to know how I can change the IP, from WiFi Router to DSL Modem?
