First things first
Take it slow, dont rush into situation, let the situation rush into you. And while most people hate "camping" tactic it's usually very effective. If you see advancing enemy coming towards you, you have to know when to open fire. No point opening fire with smg when enemy is at long range. Always remember, slow is fast. Take your time when you aim your rifle.
Learn how to handle your weapon effectivily. Smg is great when you have to clear houses or rush into flags. Be aware with your ammunition situation always. Try to use burst fire when you are engaging target.
Automatic fire works best when you have to make diversion or bluff enemy.
Rifle is good when defending area with high visibility or when weakening opposition before the final assault. Try not to engage enemy in close quartes fight when you have rifle. Always search place with some protection and good visibility. Make sure you know how to handle your rifle, it's the weapon you are stuck with in most cases.
Join a squad that plays as a team. When your squad is capping a flag never be too close to somebody cause it's good target for grenade. When attacking, stay little behind and make sure nobody flanks your squad. Make sure you have enough stamina for long sprint left and depending of your weapon rush into reasonable position. Never run like mindless zombie. Take breaks and try to make some kind of picture
of the situation.
Always take it slow