Hello,
I agree with the vast majority of the good advices that have been given.
With that said, I'm going to give you more RADICAL (not necessarily better, just a different perspective) on advice from 10-20s.
1) Don't use Benediction.
Let me reiterate. Don't use Benediction. It's worthless if you don't have competent party members that knows what a "WHM using Benediction" means. If you can't zone within 2 rounds of attack or the monster isn't at a sliver of life; don't use Benediction. Even if you ARE going to use Benediction, make sure the rest of your party has already used their 2HRs. If they haven't, you shouldn't use Benediction. It is rare to find a "good" scenario to use Benediction at that level. Usually when you use Benediction you are just fooling yourself thinking you sacrificed for the good of the group or that you saved the group. It usually isn't that dire of a situation or it wouldn't have mattered.
On the other hand, if the situation occurs where you can Benediction successfully and it's NEEDED, by all means use it. If you even save one person then Benediction has accomplished its true purpose.
2) Some enfeeblement spells are more important than healing. If you have no RDM Main/subs or other WHM subs in your group, then you must get that paralyze in before anything else. If your main tank is going to die THE NEXT HIT I can understand. If he/she's not going to then Paralyze is your top priority. Don't paralyze as the mob is running toward your group as you will attract agro. Paralyze either:
a) after opening voke or b) after 1-2 hits has been exchanged by mob + tank.
3) Your mana pool, however important people may tell you it is; is not important. Your mana pool is a threshold for "room for error". The bigger the mana pool you have the more error you can commit in terms of mana management and Healing and still get away with it without hindering the group in any way.
With that said, +Mind is infinitely more important (especially if you don't already have a ton of +mind equipment) if you DON'T RUN OUT OF MANA ON A REGULAR BASIS. There was a post here that tells you to get +mana equipment. He's Elvaan. I can understand Elvaan wanting more mana since even a competent Elvaan WHM will risk running low on mana in heavy chains.
Obviously you want the best equipment Gil can buy. If you don't have Gil, you want the best equipment left-over Gil from spending on spells can buy.
4) Healer etiquette. The conception that if your party doesn't die then you are a good WHM is WRONG. If you have what the JPN often refer to as "Safety First" party then even if you are watching TV while playing WHM people shouldn't be dying. As WHM, it is your responsibility to make sure your group should not have to wait on YOUR MANA to engage the next mob. Try to be the best WHM you can be by not only keeping everyone alive, keeping the mob enfeebled with the most crucial debuffs, but also maintaining your mana and with that...
5) Magic burst. While I don't encourage WHM meleeing at 10-20 level range, that doesn't mean you can't dish out damage. Doesn't matter if you're only at 1/3 mana when a weapon skill chain goes off. Whenever you magic burst you're getting more bang for your buck (more damage for your mana). Insist, plead, beg, or whatever to your group for them to end with Fusion (Light + Fire). This way, you magic burst with Banish (don't... MB...with... Dia...). What you do is when you see the last skill being "READIED" (Combo, Double Thrust, whatever), you start casting Banish. If your group knows what they are doing FUSION should have been activated and your Banish will HIT the target in the time window allowed for a magic burst. You will impress your group members when you magic burst as a WHM. You won't impress people who don't know what they're doing, but for them to execute Fusion in the first place they have to had known what they were doing.
Lastly
6) I do not recommend going out there and grouping right away. People don't appreciate half-gimp WHMs unless they are desperate. What do I mean? I mean your magic skills should be capped or near capped. This is not going to happen if you don't put some additional time in it or get grouped in some double WHM parties. Sit next to your mog house and get your Healing near cap. Then get your enhancement near capped (takes a lot of time at low level, I recommend doing this after you have Shell). Divination should always be capped. When you have spare (spare used here in my sense. 15/200 may be enough to Banish, but it's not SPARE in that case) nuke. Enfeeblement should easily be capped if you don't regularly group with a RDM.
Paralyze first, Silence casters next, then slow, and finally Dia.
Don't forget to cure/banish undead mobs at least once early on the fight. It is supposed to lower their defense. Whether that is true or not... I don't know.
There, that's my radical view. You don't have to take it as gospel, but I suggest you take bits and pieces along with what people have said and formulate together your own A-game WHM game-plan. Good luck.
~Nall