My First Time Playing Halo 3 Multiplayer on Halo MCC

I'm a total noob when it comes to classic Halo primarily because I have a slow reaction time, I don't know the maps well except Construct somewhat. I avoid the Battle Rifle because I'm not that good with it and I prefer automatics a lot more. Feel free to comment on how much of a disaster I am. I got into Halo properly with Halo 4 but the first games I played were Reach and ODST.