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.
See Using Force Organisation Charts for how these charts work.