Learn hands-on from someone else before you just jump into it. You can't just read a book or website and expect to know everything about installing/repairing/servicing sprinkler systems.
No two contractors install the same way. So design and install will be different on almost every job you try to service.
Do not assume that the house next door to one you just installed has the same GPM and pressure.
If you are in a cold climate, be prepared for the rush in the spring and fall. Along with the rush comes low patience levels, tempers flair, and the inability to sleep!!!
Know YOUR local codes for backflow prevention. Each state/county/city is different.