First-match checklist
Do these five things first
Join an infantry squad
Choose an open squad with an Officer. Say that you are new. Most Officers would rather explain a task than discover you were lost ten minutes later.
Check your voice channels
On the default PC controls, C is unit voice and V is proximity voice. Use unit voice for squad information. Use proximity for players physically near you.
Spawn on the squad network
Use your squad Outpost when it is safe. Use a nearby Garrison when the Outpost is unavailable. Headquarters is usually the slowest route back to the fight.
Stay on the squad task
Attacking, defending and building spawns all matter. Follow the Officer marker and ask what the squad needs before running towards the loudest gunfire.
Mark and describe contacts
Use a direction, a landmark and a short description. "Tank, north of the church" helps. A long story in the middle of a firefight does not.