For Rein, when learning him, just go as aggressive as possible and then dial it back. A good Reinhardt needs to be getting kills like the other tanks.
No, no, no. N. O. No!
For Rein you need a few things down as soon as possible to start playing him well.
1. Use his shield right. You have 2000 hp on it, it regenerates faster when it is not broken. Don't let it get broken. Hold it up to protect your team from fire from the most threatening direction. It may not always be forward. For instance, on Hollywood, I played a map earlier today where we were about to take the second point and wiped three of the team. The remaining enemies were a mercy/zenyatta in front of us, and a widow on the buildings behind. Even though I was the only one who knew that, I was able to position myself to save our teammates from the enemy widow with ease. You have to have positioning down for Rein. Not only your own + your teammates, but your enemies.
If you find that no one is standing behind your shield, you were likely not moving forward or moving with your team. Don't let that happen. When your shield is up, unless you are running into a Mei comp, you should keep moving forward if you can help it. Too often on here I see Rein players complaining about no one standing behind their shields. There is often a reason for that. Be it bad positioning, dead teammates, or a flanker on them. You are responsible for all of those. However, while you are one of the biggest sources of peel for your healers, you have to know when to trust your teammates to take care of it so you can hold the front lines.
2. The matchup of Rein v Rein is a game of patience and cd/barrier maintenance. Don't ever charge the enemy rein first unless you are point blank or nothing will prevent you from landing the full pin + wall. This is a rule you should follow 95% of the time. In the current meta, you charge, you die. Soldier, D.Va, and Hog make sure of that. If you don't die from it, your team likely has.
If he decides to charge you, you have two options. The first is to put down barrier and hop to one of the sides. The hop messes with the pin hitbox slightly, and you move fast enough without barrier to dodge it every time. You can also smack him for a free 75 damage on him as he goes past you. Your team should kill him from there. This keeps you up with certainty, but it does potentially open your teammates up to the pin, so keep that in mind. The second is to counter charge. In this situation, you should both die. Now if you are on defense, you may live due to Ana being busted currently. Don't rely on that shit. Assume you will die. You have to have faith in your team to play and win the 5v5 without you if you do this.
Firestriking is pretty simple, just aim and get free damage. HOWEVER, if he has ult this changes drastically. If he has ult, never firestrike. Other than that, you can usually throw them out whenever, just make sure there isn't a widow, hog, or hanzo ready to poke a teammate when you do, and try not to throw it into Zarya bubble or D.Va Matrix.
If there is a Rein on the other team, you have to know when he has ult. I don't care how you do it, but you have to. Rein ult is not instant. You do have time to react, but it is very demanding with the timing due to the current system. You can never let your shield be fully broken when there is another rein. Not only does it recharge slower, leading to their team winning the battle of attrition, but you have a period of at least 5 secs where you can't pull it back up. If it is unbroken, you can pull it up at any point. You need your shield to bait his ult. Put it down when you would otherwise throw firestrike and immediately put it back up. This will catch bad rein ults. You have to be tricky with this, and there is no sure answer, especially as the ranks progress. If he tries to charge or firestrike and his team is with him, ult them all for an easy wipe.
3. Learn how to aim firestrike. This takes practice, and you will never be able to hit everyone with it, but you should get quite a few kills with it in a game because of its ludicrous damage. It also charges your ult very quickly for that reason.
4. Create space by moving into positions and not just staying on point. If your team has to fight on point, you are losing valuable time/positioning. If you cap the first point on King's Row, move to the bend in the streets if you can after the first point cap to deny the enemy team their setup. You have to be doing this constantly in order to control fights the way rein should be.
Hopefully, this helps. It ended up a hell of a lot longer than I intended it to be, but I am an unintentional and reluctant Rein main of 55 hours lol.