The ABS(antilock brake system) light comes on when the ABS control module detects a fault in the ABS system. There are literally dozens of possible causes, ranging from a faulty wheel speed sensor, open/short in ABS wiring, faulty ABS hydraulic unit, faulty ABS control module, or other component.
The only way to determine why the ABS light is on is to have a diagnostic scanner hooked up to your vehicle in order to read the stored DTCs(diagnostic trouble codes). This will tell you what area is causing the light to come on, and at that point further diagnostics can be run to determine whether it is a faulty component or a wiring issue.
You can check with your local Autozone or Napa to see if their scanners can read ABS codes. If theirs can, they will read the codes as a courtesy at no charge. If not, then you will have to take it to a shop to have the computer read for stored ABS fault codes.
Once you get the codes, you can re-open this question and it will be directed to me. Post the codes here and i will let you know what they are, as well as the diagnostic procedure to accurately diagnose and repair it.