After selecting a mission, you’ll be taken to the force selection screen. Initially your units will be picked for you but later missions may give you “force requisition points” to purchase extra troops to accompany you. Each unit’s stats are shown, along with a basic description.
Notice the stars at the bottom of the screen. These represent the achievements for a mission. You can complete a mission and progress through the game without achieving these, but to win all the medals you need to achieve all the objectives, some of which are very, very tough.