Your location is important to the answer .
Briefly, if in the USA, many chain schools will give you a basic foundation course thru, 50/100 % of their Bronze social Amer.style .
There generally is no charge... however.. you will be place under a contract, not so much for teaching within the school, or training fees , but prohibiting you to teach within a specific radius of their location , for a specific amount of time.
There are many who will not employ part time staff, so check what the ground rules are BEFORE you begin.
Independant schools do not have the need ( generally ) for trainee staff .They will however train you at a cost per lesson.
The length of training should be ongoing . typically, the chain school is an 8 week course ( evenings.. w/e ) for between 2 and 4 hrs per nite .Its a good way to explore whether or no , you will like it as a career .
Your competancy level will be minimal for the 1st yr ( theres a lot to learn )
One last thought.. you must be at least 18 yrs of age .Also, do realise that most studios operate from 1..10pm weekdays and Indies are busy on W/ends.. so, if you like social life , it will become restricted .
( your name ? boy.. girl ?.. guys get booked much faster than gals, so do NOT give up your daytime job )