Wow, how exciting! :) I wouldn't know too much, but just remember how much responsibility that would-will- be. You'll need to get insurance (liability will be a big problem) and as far as horses go, you'll need a few that are solid, steady, bombproof horses for the very beginners. W/T/C steady. Then a few that are little more challenging for the riders who aren't quite as advanced.
That being said, although it's my dream to open a lesson stable, it may be a lot to take on at once- take it slow. Do you have enough money to cover tack, supplies (grooming, feeding, halters/leadropes, mounting blocks, repair supplies, etc.), building (the barn and arena itself, not to mention pastures), and insurance? Make sure you ask yourself some questions before you get in over your head.
If you plan on letting people board, you need a plan for that. Lessons? How much? How often? How many in each? Planning out everything will help out massively.