How do I start a grass cutting service??

I was a small engine mechanic for many many years. I did all the major repairs for the MS state park system for many of those years. Here's my advice:

Start with the best equipment you can afford. My advice for mowers would be:
#1: Skag
#2: Exmark

Weed eaters and blowers:
#1: Echo
#2: Stihl

Preventive maintenance is of utmost importance, should be performed DAILY.