I just started by getting piercings in the same shop, getting more and more knowledge and get to know the people working there a bit... then i startet piercing my self and others for practise (: now i'm just waiting to turn 18 so i can start working in a piercing shop (: but just get some good freinds in the business, and then knowledge and skills will come (: haha
First things first, look into what the license requirements are for body modification in yer city and state. once you know that, it will be much easier to pursue what you will need to accomplish in order to be successful.
DO NOT bother paying for a school... Most shops will just laugh and not hire you if that's the experience you give them. It's a total waste of time and money.
You need to find a good reputable shop, find out if they take on apprenticeships, and pay whatever they want to do so. This could range anywhere from $2,000, all the way up to $10,000, but it's WELL worth it, and you'll make yer money back within the first year of being licensed.
You will more than likely be working for free, being the "shop bitch," observing, and doing whatever they want you to for the first six months to a year while you learn, so it would be wise to have some funds saved up before you start.
Most places require a certain number of "apprenticeship" hours, (mostly observing) and a certain number of hours piercing/mods done under the supervision of whoever is teaching you.
AKA- "Work" normally includes both.