Experience, experience, experience.
There’s no substitute. You’ll have to accept you will lose a number of games, practice, and play a lot before you get better and start to see improvement.
Here’s the thing, even with all the experience in the highest stakes online matches, even seasoned players get nervous and jittery when they get up on stage to compete. Lots of people in the crowds watching, prizes and higher stakes, so much to get nervous about. Controlling your nerves isn’t easy, even for seasoned guys. But the more you dive in to ranked, experience the game for yourself, and play, the more you’ll come to understand the game, and realize the guys you are playing started from the same spot as you or me. We were all beginners at one point.
Experience gets us trained so we become more relaxed and familiar with the environment we are going into. You’ll still run across jerks from time to time, and you’ll have to deal with that, but most of us are genuinely good guys who want good honest competitive matches.
Also, becoming familiar with the game’s mechanics, characters, and situational awareness will really help. The more you know about how to use the tools you are given, you character, their matchups, punishment tools, gameplan, using the overall system stuff to your advantage, the less you panic under the stress of online play, and the better you do.
Finally, realize the only impression worth making isn’t related to how good you are at the game, it’s how good a player you are as far as how you conduct yourself. Skill isn’t so much a good impression as is telling someone Good Game.
Just remember there’s not really anything to get stressed about, just look for good games and get better all the time.