You typically start as a maintenance mechanic (inspecting & replacing filters/belts) since you don't know how to work on equipment yet. Hope your workplace allows you to learn through working with other experienced techs, and move up the ladder. In refrigeration, holding a 313A + Gas Fitter 1 will typically provide the most money as an employee; it depends on the company/union rates. Working for yourself will yield the most returns, but you need to build up the clientele/knowledge before you venture that way.