Start by checking out the bunker outlined in the distance; this will get you Arrows. Afterwards, make your way to the roof using the surrounding rocks.
Find the ledge adjoining the bunker. Jump on it, then turn around to face the corpse hanging from a tree. Get close to the poor fellow by moving over the limb.
Finally, reach out for the bow, which is another mini-game. Wait until the corpse is close to Lara and lean out to grab it. Your reward will be a Makeshift Longbow, your first major weapon (reward: 250 XP).