Getting better at Halo?

So I am coming back to Halo from a very long break. I stopped playing with Halo Reach and played a few games of Halo 4 but I didn’t get that Halo feel back. I am coming back again for the MCC. So far I have started to play Halo 3 again but I am just awful.
Before I quit I was like a 42 which isn’t good but I really want to push further and go to a 45+
and even go for a 50 in MCC.

So I decided to ask the forums for deep advice on how to improve my game. I want to get better with the BR so I can be on a higher level and play with my friends.

When I play with my friends I end up doing terrible and get discouraged and get off. I am really tired of that crap happening. Any ideas?