First time healing in a mmo, need tips

[CyrilLucifer]

If you don't have Swiftcast, get it. Now. No, I don't care what you're doing. Get it.

New healers also have a tendency to focus too much on curing and not enough on their surroundings. Just watch out for this and pay attention to when you do it so you can fix the problem.

Don't hesitate to use Sprint if you think you can't avoid an AOE or a specific attack. Mages are lucky and get to cheat a bit with this, so if you're coming from only playing a tank, then this will be a nice little bonus for you.

[BroodingFicus]

Welcome to the ranks. I also tanked before becoming a healer and WHM was where I started and ended up staying. Some of this has been said but I'll just list what I can think of anyway.

-SWIFTCAST: As others have said, super important. At later levels you have several 'ILL SAVE YOU' heals but even then it can be very useful when someone is about to eat dirt if you wait for a cast. Also obviously useful for raising in battle. Once you have it, you can macro your raise to first attempt swiftcast and if not available it will hard cast raise instead.

-Mind your surroundings. It can be easy sometimes to tunnel vision on the tank and step in something or miss a mechanic. If you don't use it already, put focus target on your bar to use on bosses so that you can see cast bars even while healing. I don't have a hard and fast rule about where I stand. Some fights its easier to be in close behind the boss (lookin' at you Vault) and others I stay back. Party composition has a lot to do with it for me.

-Be aware of your party. Paper tanks or DPS prone to standing in cones probably mean you should be careful with cleric. Especially at low levels where you don't have very many emergency heals available. But knowing the party will also help you plan where to position yourself for AoE heals like Medica/Medica II. For big mechanics sometimes people will stack but don't count on it. With all melee you can just run in and cast vs having a melee and a ranged which may require you to find a good halfway point to avoid missing anyone.

-Prioritize. No one likes a party member to die but sometimes you'll have to make a call. The tank and yourself should have VIP tickets for heals always if it seems certain you are going to lose someone. If a dps goes down you can always raise them.

-Sprint. Cyril said it but its still great being able to use this without any repercussions and you should.

-Stutter stepping. There is a bit of 'lag' on abilities where you can actually move just before a heal finishes casting and it will still go off. I got better a this over time but be careful early on tempting fate. I think the most deaths I've seen with new healers are ones trying to get a heal off and waiting too long to get out of the way. Unless its a do or die situation, interrupt the heal and start again once you are safe.

Other little thing you may try and see how you feel: I turn off the setting that turns you to face your target automatically when you attack/cast while I heal. Your DPS moves obviously require you to face the right way but your heals do not. Later fights that require you to face away from the boss during certain mechanics etc were made easier by this and I have less of those moments where I'm trying to cast and run but get whipped around the wrong way.

Overall just don't panic. Personally I think having experience as both main roles helps a lot with the instinct to mash buttons and hyperventilate when everything falls apart but you'll definitely have runs where you do your very best and it still goes wrong. Sometimes, as healer, you can save those runs. Sometimes not. Also since no one has linked this yet, obligatory 'welcome to healing' video: https://youtu.be/2DxS7eT_ky4