Why are you putting on an event? What will your focus will be? At its most fundamental level, what we do should be inspired by a call of God. This can come through prayer, through a heart God gives you for a place or people, because of obvious need or because you believe you need to try new things, new ways.
Decide on your age range. We started out one event by aiming it at 18-35 year-olds. We had a steady group of 30-40 people coming along and it was all well and good
Over time God revealed that we should be aiming at young people so we switched our attention to 11-16s (high schools). Even this attracted people on either side of the age range but this is the age range we mainly aim for
Remember it can be hard to get in the 15-16 year-old ages sometimes as they prefer to do other things. To get them in it has to be very cool or address a specific need or led by them etc. You can't do a naff event and get people to come back.
Church or non-Church or both
Good question. What kind of spiritual content are you going to have? Are you going to aim it so Christians can invite non-Christians along to a credible event? Is it simply going to be a Christian run 'secular event' with little or no spiritual content? Run by leaders or young people? Is it about empowering young people to learn as they go along or is it professional and therefore more likely to be run by adults with young people being trained
One church in Exeter aimed their events at local teenagers from a fairly rough estate. This was their focus group. Our focus group is our teenagers, their mates plus local teenagers we are in contact with through club, detached work or schools work. The event can be part of a city, all of a city, aimed at lots of rural churches and young people, but make it clear otherwise you won't be able to assess effectiveness!
About what God is calling you to do. Think, stay firm but do re-assess and change if that's what God calls you to do. Be dynamic, fluid but when you do know, try not to waver! (James 1.5-8)