You should not know exactly which mission you will be playing when you pick your force. To choose the mission, roll a D6 on the mission table at the start of the appropriate scenario section. Make the roll after you have set up the terrain, but before you begin deploying your forces. If you and your opponent agree, you can simply choose a mission instead.
Summary
1: Choose scenario (use strategy rating if desired).
2: Decide who will be attacker and who will be defender (use strategy rating if desired).
3: Choose your army, using the force organisation of the chosen scenario.
4: Randomly determine a mission to play by rolling on the appropriate scenario mission table.
5: Get fighting!
As mentioned earlier, forces are chosen by both players picking units from their army list up to a maximum points value decided beforehand. If both you and your opponent wish, you can simply choose your forces without restriction. However, this can lead to both sides simply going for the biggest guns and toughest troops without taking anything which really shows what the army should be like - imagine an Ork force with no Orks in it for example, just Dreadnoughts and warbikes. While this might be appropriate for a very special one-off mission, it isn't representative of the army in general. For this reason, different missions have what are called force organisation charts. These give you flexibility in choosing your units but ensure that the army keeps its distinctive character. Whether or not to use the force organisations given or make up your own, is up to you to decide, but we recommend you use them unless you are inventing a special mission of your own.