I'm a little late to the party.
If you want to go medical route, definitely go Pharmacy instead. It's still 6 years, but they are in enormous demand, and you get to help people all day long, potentially more than a doctor might. A lot of people out there really don't know what they're doing with their medications, and a pharmacist's advice can be really helpful to a lot of people. It also gives you a better sense of the people in the community where you work. I go into the local pharmacy about once a month, and the pharmacy staff knows nearly everyone in my town.
Civil engineering definitely helps people out more than mechanical. The old adage always was, "mechanical engineers build weapons, civil engineers build targets." A lot of mechanical work (being one myself) ends up working on flows through a pipe, valve, against blades (aircraft), heat transfer into any of the above, and other non-glamorous
pieces of really awesome things. Be aware that in the US anyway, Civil Engineers are
required to take the Professional Engineer (PE) exam to be certified to work. Mechanical and other engineering disciplines can get away with it as long as they work for a company. Consultants or small-business founders are required as well. Mechanical is rewarding work, but you rarely help out individual people.
Tossing this one in, one thing you may also want to consider is something like
Big Brothers Big Sisters or an equivalent program. It doesn't take a lot of time, and you can really help out a kid who just needs someone to listen to or hang out with. I can't say I do it myself because of where I live (middle of nowhere) but that may be another avenue you may want to check out if you want to help people.