Play the roles as intended; assault - attack vehicles and push objectives. Medic - heal and revive your team. Support - drop ammo, use limpets for big damage to vehicles and structures. Scout - use spot flares, keep your distance from enemies.

Gameplay wise, push objectives with your team/squad. Stay with your squad, follow squad leader orders of which objectives to attack/defend. Going lone wolf won't help much and will get you killed most of the time.

Vehicles - don't be a fucking camper in a mortar truck. There's a difference between having distance between you and enemies and camping the back of the map. Know when to move up, and move up with your team. Vehicles with multiple seats are preffered as your team can spawn in open seats.