Everybody has to learn to play the game - most people understand that. However, new players also have a responsibility to communicate this, and also realize when they should switch to a more comfortable character.
It's amazing how much more understanding someone is to a new player who is actively communicating. Starting out a match with "Hi, I'm a new player with Reinhardt - let me know what I can be doing better" works wonders, as it gives more experienced players a heads up that you might need some guidance or some extra support.
New players also should try to listen to constructive criticism and be willing to cut their losses. If you've been playing a Reinhardt and someone 50 ranks higher than you says "if you see Torb setting up a turret, launch a fire strike at him" - acknowledge that somehow, even if it's a "thanks" emote - and DO IT. Also recognize that you probably suck. If another person on the team says "How about I role RH for a little bit" - move aside! You aren't forced to practice new characters the entire match. If things aren't working this match, it is often best for the team if you switch to a character you are better with - if you're playing a tank or support, let the team know though!
An example of this: I'm trying to learn Zarya. There have been a lot of matches, especially early on, where I've been absolutely horrible with her. I'm pretty sure a Mercy solo killed me once. When I recognize that "Wow, I want to learn Zarya, but these opponents are just too careful around her" - I switch to a character that I can contribute with. Over a dozen matches starting with Zarya and then backing off when I need to, I finally have become competent with her. You can learn characters in bursts.
Always keep in mind that this is a team game. Experienced players should absolutely be understanding of new players, but new players also should realize that if they're having a particularly tough match with a character, switching off for the rest of the match can be the difference between your team losing and winning.