So basically, there are a few puzzle pieces out of which one can build a guard army that determines how it plays. At 1500 points or less, pick up to two of the following:
infantry
mechanized
armored
air cav
artillery
above 1500, you can add a third if you want, but it's not required.
Each of these 5 things has a few shades of nuance. For example, mechanized has mech-vet, and platoon-based mechanized with or without al'rahem, etc.
For me, I have a big pile o' infantry at the core of my army. It has often been trimmed with artillery, although I'm thinking of doing something else in the future.
This is how it's done in the field.