Generating the Regiment
You have 12 points to build the regiment. You guys should come to an agreement of what you want it to be, as all your characters will belong to the same regiment. After the regiment has been built you can move on to actual character creation. Some details on the choices can be found in the spoilers.
Agree on an Origin
Homeworld | Cost
Death World | 3
Fortress World | 3
Highborn | 3
Hive World | 3
Imperial World | 1
Penal Colony | 2
Penitent | 3
Schola Progenium | 3
Personality | Cost
Bilious | 2
Circumspect | 2
Choleric | 2
Fixed | 1
Maverick | 2
Melancholic | 2
Phlegmatic | 1
Sanguine | 2
Supine | 1
Regiment | Cost
Armoured Regiment | 4
Reconnaissance Regiment | 3
Drop Troops | 3
Hunter-Killer | 3
Light Infantry | 2
Line Infantry | 2
Mechanised Infantry| 3
Siege Infantry | 2
Training | Cost
Close Order Drill | 2
Die-Hards | 3
Favoured Foe | 3
Hardened Fighter | 2
Iron Discipline | 3
Sharpshooters | 4
Survivalists | 4
Equipment | Cost
Augmetics | 2
Chameleoline | 3
Combat Drugs | 2
Demolitions | 3
Scavengers | 3
Warrior Weapons | 3
Well-Provisioned | 3
It is ok to have points left over, as every point not spent will add 2 points to the Additional equipment, which can be worked on as a group last. Additional equipment has a ton of options so this part will require more discussion. The expert 40k players and anyone with access to the book should probably take the lead for this part. Your base for Additional Standard Kit is 30 points.
After the regiment has been created, work together on fluffing it up, then we can move on to actual character creation. Other than picking your specialty and spending 600 experience for Guardsmen and 300 experience for Specialists, then fluffing everything, creating the individual members will be easier than designing the regiment.
Once the regiment has been created I'll post the official rules for generating your characters.