This thread is for Beginners with the DMR, or people who want to warm up or improve a bit. So some of this information will be obvious to some people viewing this thread.
Firefight, try playing this playlist more. It will help you on you're situational awareness. Get yourself used to the bloom effect of the DMR and how it handles in close, medium, long ranged combat. Take you're time no one is rushing you since it's only just firefight got plenty of time to work on the aim with the DMR. It also helps you're reaction time, i have been doing this before i warm up and play Multi-player matching everyday. I also learned this way as well. When you start getting better, move on to Matchmaking and do not let people bother you, start off with a playlist you might/will enjoy.
Close range with the DMR best way to use it. Spam you're sots and on the last two shots bring it up the head.
Medium range moderate paced shots. Don't get anxious and start spraying like you can in close range.
Long range, take you're time this is the hardest range to kill someone if you do not have a Sniper rifle. Takes practice to get kills at long range very decently because you're shots will miss more than the other ranges obviously.
Always, take advantage of a player to when you can. Aim for the chest area, as more shots will impact the body and the last shot you just need 1 clean shot and you will obtain a kill. You're first shot will always be were you aim, then it blooms out and where the circle is spread to it will go anywhere in that circle, thats why pacing is crucial at longer ranges. Throwing Grenades and shooting a DMR at a opponent is a good tactic, but don't spam grenades waste of effort and time and valuable grenades you could have done better with. DMR + plasma pistol seems noobish, but works great for faster head-shots.
When you get really decent with the DMR, move to Team Swat. Be warned, this play-list is 1 shot kills to the head and you have no shields at all. It takes very good reaction time and aim, and you have to be aware. Good Play-list for you're head-shot and precision weapon commendations as well. Once you get good at Team Swat, you have achieved great potential, and always practice even after you have accomplished all of this. Warm up before you get on Multi-Player Matchmaking so you will have a feel for it better an get in the swing of things faster.