What is the rhythm for you? Trying to learn her because I somehow seem to be trying to learn all the tanks.
Honestly, it's something you've got to feel out. I'm still working on it myself
You want to be popping it just before you pop into view, or when you know an attack is coming; nobody's going to shoot at you when you've got it up (not after they get a taste of what happens next), but that moment of "movement, fire" can give you some sweet charge. As for the second, it's about knowing the enemy team comp, seeing what they're doing. You see a reaper try and sneak into the middle of your team, wait 'til you hear the first "die" and turn it on. Dive into Hanzo's dragon and pop it. Stuff like that. There seems to be a per-use cap of 50% charge, since that's where it seems to top off a lot for me, but I
think I've gone higher than that before, so it might be source dependent? Either way, once you hit 50%, it's not the worst idea to start thinking about safe place to run to when it drops, since that's one of the few times the enemy team is gonna lay into you with no reservations, and you're a little squishy for a tank.
The teammate shield is harder. Throwing it on your support is never really a bad play, since they're always going to be a priority target. It builds charge through Reinhardt's barrier, so shielding him up is a good idea too. Having teammates who communicate and know to push when they've got it is helpful too, but not really an option during pub games.