You will automatically get the bow in the beginning of the game. You can equip it at any time by visiting a vending machine (store) in one of the liberated Garrisons. A dragon spawns almost always near the Garrison with the coordinates X: 437.1, Y: 505.6. That's the Garrison on the very left, in the north-western part of the map. You can see the coordinates on the in-game map. Go there, disable the defense system and throw a heart to attract the dragon. Hearts can be obtained from dead bodies. There is a turret in the Garrison. Use it to weaken the dragon until the health bar is barely visible. Then switch to the bow and shoot the dragon a few times until it dies.