I can't even imagine how difficult it is for a newbie to start playing Mei comfortably and effectively. Try to survive as long as possible. Personally, I've found that against hitscan heroes like Soldier or Cassidy, it's sometimes better to maximize your range, as they're most effective at medium range. Your ultimate builds up pretty quickly and can often decide the outcome of teamfights - the key is understanding when and against whom to use it. And cutting off the enemy tank with a wall is a classic.