To be honest, there is no "one stop shop". The best place to start is going to be dependent upon where the event will take place. Public land = city, county, and DNR. Private land = City/ county (and still possibly the DNR, believe it or not). Most places have several permits that need to be pulled (for number of people, for alcohol, for open fires/ bbq, for noise, for parking, etc). These usually all have a fee associated with them.
Along with the permits is insurance. If it's private land, homeowners usually won't cover any damages or liability for an event that is open to the public OR is charged for. Best route is to have everyone sign a waiver as they come in, but that still won't cover your butt. I know local placed have to have event coverage of $1,000,000 liability if they are serving alcohol, so that's something to keep in mind.
Honestly, you're probably too late to go about this the legal way. Sad to say it, but for events like Snofari and Sno-blind, we start pulling event permits this time of year to host the event in January.