Changing pads is a simple thing usually.
Now, the light. When you said the brake light, there are two lights that you could be referrring to.
1. The red brake lights at the rear of your vehicle.
If this light is coming on and STAYING on, there is a problem with your Brake Light Switch or Stop Light Switch. Only difference between the two is the name (as in, which part of the world/country you live in). The brake light switch is located at the bottom of your brake pedal. When the button is pressed, it turns the brake lights off (your pedal pushes it in when it's 'at rest'). When you press on your brake pedal, the button is released and you complete the electrical circuit (think of flipping a light switch in the house) and the brake lights light up as they should.
2. Your ABS light on your dash.
If you have an Anti-Lock Braking System, when there is an issue with THAT system, your ABS light will come on and stay on. An issue could be a simple malfunction/defect in the electronics of the system OR it could be the brake pads themselves. If you installed pads that were not designed for an ABS system.... that would be an issue.
#1 can be simple enough that you can do it yourself
#2 is an issue that I would take to a mechanic that specializes in Brakes.
If you had a professional (licensed shop) mechanic do the work for you, LET THEM KNOW what issue just happened. MOST mechanics pride themselves in their work, especially if they value their reputation, and want you to have a vehicle that is in proper working order that has their name on it for repairs.
If you did the work yourself or a 'shade tree' mechanic (and not all are bad, some DO know what they are doing) was the one who did the work for you, let them know as well. If they say 'i don't know' or start to scratch their head, seek out a professional shop and be prepared to pay the money that you should have paid to begin with.
If you can recoup any of your costs from the 'shade tree', congratulations.