In Call of Duty, keep moving. Its a run and gun shooter.
Keep your eyes open. Keep scanning the battlefield in front of you for enemies. Don't let your brain go into autopilot.
Don't panic when you encounter an enemy. Many people new to shooters suddenly panic when they actually have to take the shot, and start spraying everywhere. Keep cool, focus, aim down the sights and pull the trigger.
Use grenades to check corners, rooms and hallways. When your grenade hits an enemy, you will get a hitmarker. Throw them into rooms and around corners to "check" for enemies.
Read the perk descriptions and pick the ones you like. The perks are very helpful.