I guess I need to learn what a kit item is and how to sue them?
The best classes for infantry teamplay:
Assault (primarily a medic) = medkits/medpacks, revive ability
Support = ammo packs, ammo boxes
Just throw medpacks and ammo at people, even if they don't ask for it. If there's a pause in the fight, drop some. If you are in a fighting position, drop some.
Then the second tier of team-orientation:
Engineer = repair tool, anti vehicle weapons
Recon = spawn beacon, sensors to help spot guys on minimap
These classes require more finesse to be useful to your team, although they seem straightforward at first. The gadgets don't work in any easily explainable way, so watch other players and read some guides. A well placed beacon can turn the tide of a map, and having enough anti- vehicle weaponry on the field is important for some maps.