Make, model, year of car???
Which light is on (check your handbook)?
Generally it is best not to drive the car if any warning light is on.
Get the fault code(s) read by a garage with the relevant diagnostic equipment. On some cars they can be read using DIY methods, but we dont know what you have.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.