Hey,
Well my first piece of advice was going to be start going into MLG by yourself (a tip given to me by many people who are better than me) BUT it turns out that, just like me, you're fed up with worthless teamates who go mass negative, either don't have a mic, are in party chat, or don't use their mic, don't listen to callouts or use any form of teamwork whatsoever, and betray you for power weapons that they don't know how to use.
Instead, you should try playing some customs with some of the regulars on the site (sorry about the wall of text, I just lost 4 straight games in MLG and need a place to vent

). This way, you're gauranteed to get good teammates who communicate and will help you improve by telling what you did wrong and giving you suggestions on what you could've done. You'll also get used to playing at a higher level than what you're used to and will learn to think and strategize on the fly, not to mention your aim will improve as well.
My G.T. is "Ponked" and I can give you a few other names of people who I don't think would mind helping you out either so send us all friend requests.
Some other names:
blackout bobxxz
Ashthera
Invincivil
Not sure about these, they might not accept your friend request but try them anyway and say Ponked said to add them, that'll be sure to get you declined

:
NA Golfball
Thier Green
Simply Electric