Complete missions to unlock the next in a chain. You'll notice I have completed the first two missions here, one time each. Repeating them will give me new rewards each time. The current reward for completing the mission is pictured on the right of each mission window.
At the top of the screen you can see what your next mission objective is, and the yellow circles off to the right show the next places you need to go. For this objective, you simply send your group close to the truck, then blow up the barrels.