Plumbing , HVAC , and electrical . In that order . As a Maintenance supervisor in Atlanta Ga , with over 20 years in the field , you must be proficient in all three . The majority of the repairs we see involve toilets ,sinks and piping.An individual who can master these skills and also have an EPA certification will receive the best salary . Take a course at your local college or trade school . If you are currently employed in the field , watch ,listen and learn from those with experience .Ask questions and show initiative . These are the things that elevated me to my current position . Now if I could only find a job doing this in the tropics I'd really be happy .