Well 1) if this is your FIRST healer i would say lvl as a healer .... it wont be as fast and you wont do nearly as much damage as you would a shadow priest. I have found that 70s who lvled shadow and then switched to healers just aren't as good (in my experience anyway). Plus, it is a game first and foremost so if you want to lvl as a healer you should, i went shadow with my first priest and it was fun but not really what i wanted to do...so i guess just do what you want to do.

2) if you do want to go  Healing (dont go only holy)... i would mix it up between Discipline and Holy ( discipline because of the mana benefits along with some other things are always nice like improved bubble and wanding.... priests are squishy... they need all the help they can get (lol) ).  ~ the ratio to which you mix these 2 specs depends on your style of healing, i have met some who go so far in disciple that there bubble does damage and i have met others that only go up to the spirit buff (which i did/do), just play around with it.

~ oo... and side note dont be upset if you get spirit of redemption until later, the truth of the matter is it only actually saves ppl 50% of the time (and that is being nice..if i had guess for my life i would say much lower but o well...not to mention you can only use it when you DIE and theoretically the healer doesnt get hit)... if you have better mp5 or more mana you have a much higher chance of being a successful healer (again.. this is only my opinion feel free to disagree)

and so yea... i am not exactly sure if this was the answer you were looking for ... but at 1 am thats the best i have...so gl with your priest! : )

(and sry for the grammar mistakes/ prolly spelling mistakes to :p but again 1am i really couldnt care less)