It requires:
1. A tree board or department
2. A community tree ordinance
3. A community forestry program with an annual budget of at least $2 per capita
4. An arbor day observance and proclamation
Pretty simple stuff, but #2 and #3 can be hard to get accomplished without significant help from local leaders. I work with 3 tree cities, and each one is very different. Ohio has regional urban forestry coordinators who are available to help with the process. Perhaps MN has something similar?