My car won't start, have a new battery (less than 3 mo. old) an it won't jump st on 2003 Cadillac DeVille

dash lites come on, headlights too, but starter won't engage, and nothing inside works door locks windows radio seats etc.

Asked by for the 2003 Cadillac DeVille
You need to remove the negative and positive cables at the battery. In that order. Remove the rubber covers and pay close attention to the negative terminal for excessive corrosion. Clean till all stuff is off terminal. Reconnect positive first then negative.
1 more answer
Does your security light on the dash come on and stay on or flash while trying to start and or leaving the key in the on position? If so try turning the key in the crank position, let off and let it set in the on position for approx ten minutes, making sure you have a strong battery and watching the security light. If the light finally goes out, try starting the car. If the car starts, the car was in security mode. You may want to have this problem looked at by a trusted repair shop.