Adrenaline move to heal multiple allies that have lost HP that won't be easily covered by rapture regens. Worth mentioning also good to use if multiple people in your party get hit by range LB, range lb applies healing reduction, but Healer LB ignores this debuff. It's mana efficient to use it as a result in circumstances like these examples.

Healing in this game probably more than any other role comes with experience, you just have to play a lot of games. Onsal healing is a bit more important than Shatter for example, where there's less skirmishes and more making sure you help your DPS by glaring the ices and keeping miseries. Seal Rock you'll help with DPS in mid on the adds, and then support your party with base control.

In PvE as a healer you want to minimize healing GCD usage and maximize damage uptime. On WHM, this concept in PvP is similar in some scenarios, but you are a healer first and people take more damage and more regularly, so make sure you drop some old PvP habits and prepare for some good old fashion healing. However you'll want assists and any kills you can snipe to build battle high; so don't forget to use free gcds on glare and dump your miseries everytime you have a blood lily.

The best discord for frontline tips and to ask for help, advice or even people to play with is probably Revival Wings - https://discord.gg/wWrnBcM (invite code)
Brian Ricardo also plays with viewers when he streams so it's also worth playing some games with him when he's doing Frontlines - https://www.twitch.tv/brianricardo