The ABS ligth is on how can I know what is the problem?

See my other answer to your IDENTICAL QUESTION.
It is only necessary to post your question ONCE.

First of all have the ABS scanned and check the history for codes that would zero it down to which one it is then check the wire for loose connection or corrosion.

Well you can drive without ABS the brake light came on because the ABS. I would have it serviced they might tell you the ABS unit is bad you don't need ABS they just prevent your tires from locking up during heavy break use

Hi these trucks are know for the front hub bearings to fail., the wheel speed sensor is located in side of them and serviced as a unit. Mostly the set the abs light while doing slow turns your will hear the abs motor and feel the break pedal pulsate.

based on the age of the vehicle most likley the ABS module failed. Only way to know for sure is to get it diagnosed. There is nothing you can do at home without the proper tools. And no way can anyone guess accurately without testing. Codes are needed to proceed.

Yes, The light should clear after fix the problem. Check the connection on the sensors. Theres 4, one at each wheel. If you have air in the lines the ABS light will come on. To bleed go left rear, right front, right rear, left front. Don't let the reservoir go dry. Brake fluid should be changed every 30,000 miles

IF YOU ALREADY HAVE THE ABS FIXED THE LIGTH AUTOMATYCALY TURNS OFF BUT IF THE ABS IS STILL UNABLE THE ABS LIGTH AND THE TRAC OFF LIGTH WILL REMAIN ON

That sounds like the ABS pump. If you have 4 wheel ABS that pump has a history of problems Leave it alone if its working now. Pump is very expensive.

sounds like a dodgy ABS sensor (change both) but while your there also check the condition of your brake pads