Each stage features 4 medals on easy, and then another 4 on hard (which unlocks after earning all 4 medals on easy). The medals for each stage are:
-kill 70% of the enemies
-kill 100% of the enemies
-rescue all people
-don't get hit during the stage.
You don't need to get all medals during the same try, you can go for them one at a time. My advice would be to get all medals in stages 1 and 2 and some in stage 3. This is obviously much easier with upgraded gear so be sure to spend stars on obtaining more health and better weapons.