PvM Support
I would not recommend anything in this build until at least level 90 high priest, unless you have friends that you want to level with. However, if you do level with this build as a priest, here's the skills you will need:
Increase SP Recovery Level 4
Resurrection Level 4
Impositio Manus Level 3
Suffragium Level 3
Aspersio Level 5
Status Recovery Level 1
Kyrie Eleison Level 10
Magnificat Level 3
Gloria Level 3
Lex Divina Level 5
Lex Aeterna Level 1
This will leave you with 7 skill points leftover at job level 50 to do whatever you want with. You could get more levels of gloria/magnificat, 2 more levels of impositio, etc. Also, you can get rid of gloria, suffragium, or aspersio if your particular needs do not include them. I will go into further detail as to how to use the skills and what situations call for what skills later; for now I am just giving bare basics and a WoE guide. As far as equips, in most cases it will not matter. Get a shield card that gives a racial bonus against whatever you are fighting. You can get either dex equips or defense equips; for support it does really matter, it depends on your preferences. You should get equips that supplement your build; if you prefer builds with very high dex for support, get equips adding dex; if you prefer to tank the monsters for your party, get defense equips. Support priests are very flexible. If you are a dex priest, do not try to tank things; as high priest you will be able to even with low vit, but as a normal priest your max hp will be fairly low and you won't have assumptio.
As a high priest, I recommend the following skill build for PvM support :
Increase SP Recovery Level 5
Resurrection Level 4
Impositio Manus Level 5
Suffragium Level 3
Aspersio Level 5
Sanctuary Level 7
Status Recovery Level 1
Kyrie Eleison Level 4
Magnificat Level 5
Gloria Level 5
Lex Divina Level 5
Lex Aeterna Level 1
Assumptio Level 5
Meditatio Level 10
You will have 4 points left over at job level 70, and the skills that were optional as a priest depending on your needs are still optional as a high priest. Your stat builds can afford to be a lot more varied as a high priest, and hybrid support builds with high dex that can tank are more viable. Still keep in mind that you will not always be able to handle every situation, so if you're going into an area with strong monsters it's nice to have some healing items to fall back on if you need them, and fly wings in case you need to get out of an area fast without worrying about delay on other skills.
The best way to learn how to support any given class is to get some experience being that class; then you'll know more about what they need, and when they need it. Here's a list of classes and how you should be supporting them.
Turn Undead
Increase SP Recovery Level 5
Safety Wall Level 7
Resurrection Level 4
Impositio Manus Level 3
Aspersio Level 4
Sanctuary Level 3
Status Recovery Level 1
Kyrie Eleison Level 4
Magnificat Level 3
Gloria Level 5
Lex Divina Level 5
Turn Undead Level 10
Assumptio Level 5
Meditatio Level 10
Basic TU skill build. Will go into more detail later. SW is completely optional; I would not get it unless you're job 70, it just allows you to do more things without having to reset every time you want to level/leech others.
For stats, pump dex up to a medium-high level, get 8X int, 30-40 luck, and low-medium vit. Kill Evil Druids until the higher 70's, and then stay anubis to 99. Anubis gets a lot easier with demi-human resist, but it is very doable without any equips whatsoever, or full npc equips. It is one of the fastest places to level.
You can also head to abbey level 2, and turn undead ragged zombies for tidal shoes. The zeny isn't that great anymore, but you can overgrade the shoes to use yourself or sell upgraded shoes for a decent amount of zeny if you're lucky. Magnus E is still a faster build to earn zeny with in most cases.
http://www.youtube.com/watch?v=VrQXf9fZHu8" A TU Priest killing anubis; I recommend using lex divina on anubis before you get to high priest, as in most cases their sonic blow will one hit you without demihuman resist and decent defense or assumptio.