Civil and Mechanical are both great branches of engineering. So basically it depends on your interest and potential for the same. But I would suggest you to go through the below points before making any decision -

Mechanical Engineering -
It contains a lot of major topics like IC engines, Thermodynamics, robotics, material science, aircrafts, etc.

CIVIL ENGINEERING
Civil engineering is basically focused on construction work as well as development of the structures.
It will consist of different types of structures, various loads acting on structures, development of different techniques to make buildings more reliable and stronger, etc.


Hope this helps!!!