WHM is the Healer class it's what you are mainly good at. DPS as a WHM got a boost with some new skills also in the latest patch though we lost some of our capability to mitigate with a stoneskin nerf. But you can pump out heals to an entire raid easily and constantly keep HoT up. You have emergency heals and simple single target cures of various strengths as far as healing itself goes WHM is the most capable.
Playing any sort of healing or support class you should know anticipation is key. Learn to love focus target and how to read the skills coming up. After a while you will learn the orders things happen in and can plan accordingly. You need to be learning about your party. When does the Tank need healing? When will the whole group take damage? Who is struggling to dodge? Does the Tank lose aggro to a DPS?
It's a good idea to read up on the skills of various Tank classes too so you can understand when they are using a defensive skill. If they are in trouble and you know they already used their big CDs you might have to burn your benediction CD to save the Tank.
You just have to keep learning fights and different situations then you'll know how to counter them. Develop a thick skin because there is a stigma that Healing is really easy and people will just blame you for almost everything unless they are very experienced and can see who exactly was at fault. Sometimes it's your fault and in that case hold your hand up and say you made a mistake but the rest of the time don't let people convince you that you are bad because you can't carry them.
When it comes to the Healer people want to take it's one who they can rely on to do the job so they can all do their own without having to worry. A reliable Healer and Support (Basically only Bard is support Machinist has support but nobody seems to know how to use it right yet) can make the run that much more enjoyable for everyone.