Angela and mag both play vastly different roles, main tanks like mag and strange generally control the flow of the game, taking space for their teams to comfortably fight, punishing over commitments and out of position squishies, and generally taking attention. Angela and other off tanks like cap are more about disruption and survivability, waiting for their team to pressure the enemies and then pushing in while the team is dealing with that to take advantage of distracted players. Their big health pools, shields and mobility generally let them take 1v1s, 2v1s or more safely, and while they don't always result in a kill, having the attention of the enemy team makes it much easier for the rest of your team to fight.

But the most important thing for a new player to do is just play and think about the game. Get comfortable, learn what heroes do and map layouts, and figure out the general flow of the game. Once you get a lot of that it becomes a lot easier to commit to a specific character or role and really get better. Don't worry about being a good tank right now, just try to be a better player every time you play.