Marvel vs. Capcom 3 for Xbox 360 - how to get better at the game?

So I've been playing for a while on arcade and I feel like I'm pretty good. I've tried it on very hard and I got up to like the 4th battle. I recenty got Xbox Live and now that I'm playing against real people I suck so bad. :P I've done like 100 Xbox Live battles and I've literally only won 15 times. :( I was a 9th ranger then went down to fight then 9th ranger again and now I'm once again a fighter. They always pair me up with people I have no chance against though like 9th lords and judges or whatever. Also I've even fought the highest rank but I forgot what its called but I know cus its an unlocked achievement lol