To progress in Hover and unlock new content, you will have to win rank. The rank is shared by your entire team and there are many ways to improve it.
Scan the city in order to find non-playable characters who can give you missions. Many NPCs in the City can offer you themed quests that unveil storyline pieces. Most of missions can be completed without defined order.

You can also freely explore the city and achieve challenges without starting a mission or a speaking to an NPC. In the HUD top-right corner, you can always see the challenges available in your area.