[Rollos]

You are probably throwing grenades too early. You should pretty much never throw a grenade during a close fire fight unless it's 2v1. You are also probably not using your grenades effectively. Pay attention to your radar and bounce a grenade off the wall when they are coming around the corner to take away their shield.
But the most important thing is to stick close with your team. Most people don't understand that Halo is a team game. You can't run into a group of people and take them out by yourself.
Cheap tip: Get in a ghost as fast as possible and proceed to run people over.