Your kit needs to be packed according to your skills. Filling it with equipment and supplies you are not qualified to use needlessly takes up space and could open you up to some liability issues. The Good Samaritan Act only protects you if you only provide care according to your level of training.
That being said, start with a bag. How big a space do you want it take up? I tried some of these
Statpacks at the last EMS conference. They are very well built and well compartmentalized. They are a little expensive though. If you decide to go that route, I'll get you some local contact info.
What training level do you have? Combat medic? First Aid? EMT? That will help me give you a better opinion on what to stock in your kit.
Here's an overview of my trunk kit, but keep in mind I also use this while on duty:
Large Trauma Dressings
Abdominal Dressings
Sterile Burn Dressings
Asherman Chest Seal
Charcoal
Glucose
Sterile Water
Saline (for irrigation)
Kerlix
Ace Bandages
Sam Splints
Assorted guaze and bandaging materials
Vaseline Gauze
An oxygen setup, with nasal cannulas, NRB masks and an adult/ped BVM
An intubation kit
BP Cuff
Stethescope
I'm sure I'm missing something but those are the materials I access the most.