They automatically spawn. You can go in and set garrison points, and research upgrades for faster spawn rates, but you don't have to do anything to start the spawning. Although I believe you do have to manually set it to spawn one of the goblin types (prowlers, I believe), to switch it from the default raider spawn.
These units, along with your laborers and/or marauders, will be in constant production until you reach your population cap.
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"The sky grows lighter my friends. The break of day should cheer the heart. But for us, sunrise brings only bloody war. Make thou ready. Defend my people well." - Prince Erian IV, Dawn of Fantasy