You can start a raider career only if your character has reached level 150. That's when raiders first visit you at your base and offer you some tasks to do. Raids are only available for experienced players, capable of both attacking other bases and defending their own.
If you've already reached the required level and summoned raiders using the radio, but they aren't showing up, contact our tech support in the game settings.
As you participate in raids, you might get nice resources in exchange for your own active engagement and risk when making such forays.
Good luck in your hunt for resources!