Sprinting takes getting used to so don’t worry you will learn it with practice.
MG’s are helpful when you can’t aim great, but one thing I have seen new guys do is just unload the entire clip even if they are not pointing on the guy (that is not smart especially with the AR, it is a very accurate mg when used right). My only advice would make a shooting range or practice course on forge and try stuff with it, or just play really, you pick up habits once you see an enemy do the same thing over and over, so test different strategies that work for you.
I generally just glance at my tracer when I see red and that is it, it is helpful at times but it can also confuse you into getting in the one spot the enemy can kill you. Best thing is when you see a red dot just check around more cautiously to maybe find him before he finds you, you will still die a lot before you get the hang of things (unless you are an amazing halo talent) that is part of shooting games in general, just be patient and learn from the mistake you made before.
Then don’t panic, slow it down move your aimer slower (not slowly just slower) until you see him and try to keep track of him, good way to do this is forge a bock a about the same height as you and place a block or cone on top of it than do circles around the block keeping your aimer (without shooting) on the cone or block for a while than shoot it when your ready. That is not a guaranteed method but it helped me, also in those situations you need to conserve ammo in your clip, with an mg you just need to be In his direction maybe not directly on (although that does help) but with rifles defiantly do not just shoot randomly, wait till your certain you have a shot than unload on him. Always be moving, it is much harder to get shot when the other guy has to adjust to you as well, because your aimer moving and not you is an assassination waiting to happen.
Just practice with the other weapons, find one you like and get good habits and techniques to use with it. I have no real good advice for this one because I am not sure how it all works for you.
If they are coming at your jump to the forward and to the right or left at the same time and you can dodge some splatters, than in the air turn your aimer around and sprint as soon as you land. It may not work great but it is something hopefully that helps.
If you are close range and have a rifle it is very handy to sprint and hit the guy to take his shields, than just go for the headshot or unload body shots. Wait for the right opportunity because it is easy to miss and than get hit right away, this is very prone to trying for assassinations it just takes practice. I have a thing called elitepocalypse on reach where it is only elites and I am unlimited health, than I just practice my assassinating skills there. Get a method to practice with and use it, for some hitting is not there favorite they would prefer to shoot than risk the loss of shields and that is a good method to so find your favored way to play and use it.
I know this is not great advice but it might be something helpful, I am a little new to halo so more experienced guys might be able to help more than me (for sure). Hope it helps