I think AST and WHM can both be friendly to new players. Coming from a DPS background AST might feel a bit more fulfilling because of the cards, WHM turns into Stone spam pretty quick.

The best advice I can give is just stay calm. Keeping a level head can help you get through situations that would have otherwise been impossible. Whether its avoiding a wipe because the tank went down or handling a rude party member it's your biggest asset as a healer.

Flat out, not to discourage you from healing, you will get blamed for things that aren't your fault. It's okay, it rarely escalates, just don't let it get to you.