You can use your ice walls for mobility, this is very important for the missions where you're racing the enemy team to battle for a point, and it comes in handy plenty of places. Practice bringing your Ice Wall vertically in front of you to lift you to high spots, then click R1 again to break it immediately to get it charging and not get in the way of enemies.
You are D.Vas worst nightmare, when you're teamed up, harrass her whenever you can. In fact, harass most tanks in groups. 1v1 you can take almost anyone, except Torjborn. Tracer, Sombra, and others with teleport skills are difficult, but if you can time it right you're good. Reaper is difficult, but try to wait to confront him until you're sure he doesnt have that invulnerability thing charged.
Try to separate healers and support from the rest of your group.
Use your icicles. This is something I didn't get at first, I thought the frost ray was the most effective so I spammed it, but your icicle is really powerful, with good enough aim, you can harass Pharah out of the sky and take out Snipers from across the board. If you feel it necessary, you can even climb high and snipe yourself (this is where the mobility helps) but this is best when on defense and you know there are other snipers that want the high ground, so you can harass them down then claim the high ground for yourself.
Your ult is multipurpose. Use it for a push when with allies and enemies are grouped. Use it to keep enemies off the point when your team needs to regroup. It takes a little practice noticing good places to Ult, but you'll get the hang of it.
Any other questions, feel free to ask, I'm still learning Mei myself, but these are things I've both been told and have learned myself that are really effective. You are insane when you have support on defense. You are a tank killer with support. On your own, you can solo almost any character. The key is being flixible. When on defense look to hole up with your allies, and when on attack try to stop lone flankers. Overall, practice makes perfect, Good Luck, and I'm sure you'll wind up A-Mei-zing
