Old or new slots?
For current models, I'd say the best thing is look for casinos in your area... if you have any Indian casinos, etc - and see where their slot techs went to school.
The only other option is move to the closest trade school for slot techs.
Problem is you're likely not going to learn as much, or anything about the older stuff that is discussed here, if that's what you're interested in. Many quirky problems can be brand and/or model-specific like old CEI video poker, or old IGT, Bally, Universal, slots.
If you have any electronics background, that helps considerably. Knowing how to determine power inputs and outputs, checking fuses, wiring, diodes, capacitors, etc. Error codes show malfunctions and can lead you to the problem areas, etc.
NLG is a humongous source of information once you find it. We have an entire other site "newlifegames.net" that is full of information on older slots. If you are a contributing member, you have access here to hundreds of documents, manuals, wiring diagrams etc.