Engine Ligth On - Found the gas cap not on tight

Engine Ligth On - Found the gas cap not on tight. How do I remove the light or is it just as simple as taking the - post cable off? I all so found that the engine has been changed I that vehicle and the diagnistic port is not connected. What is needed here. I just got the vehicle in NC and I am in Port St. Lucie, FL now ! Iam retired and Vet so the first is the best time for your donation so the address here is Milton Baskett Jr.
1192 Lombardy Dr.
Port St. Lucie, FL 34986
772-340-7818
cell ph # 919-702-2428