A school in the US, I presume from your language & location info. The short version for Part 61:
* Get a flight instructor certificate appropriate to the type of instruction you wish to do
* Get access to an appropriate aircraft & maintenance for it
* Get liability/professional indemnity/accident insurance
* Get access to some sort of facility(s) for any ground training & theory exams
* Pick an airport(s) & hang out your shingle. In other words, get some students.