Civil and Mechanical Engineering are both broad, which is a good thing. Civil Engineers basically work on things that do not move (buildings, bridges, roads etc.) while Mechanical Engineers work on things that move (planes, cars, motors, etc). It really comes down to what you want to do. I suggest you Google both types of engineering (and they are many off shots of each one) to see what you might like to do. Also google "jobs for mechanical or civil engineers" to give you and idea of which industries they work in. Many industries hire both types. The first year of engineering school for both civil and mechanical are very similar so if you started in one and then decided to switch to the other, it would be possible.
Good luck, hope this helped.