It's also important to understand the basics of the elements before you make your mage. Fire does the most damage, ice does the second most and slows the enemy down and light does the least but can stop the enemy in its tracks sometimes. There are 3 more knight noah common combinations of this. Fire/ice, ice/light, and light/ice. Fire should not be used as a secondary because it's only for doing damage. As a secondary skill, you won't get the better  skills until much later, so the damage wouldn't matter anyway. If fire is your main skill, most people like to slow the enemy down enough with ice so they can hit hard with a fire skill then run back and attack with more spells. If ice is your main skill, you don't want to use fire as your secondary, leaving light as your best choice. The same goes for light mages.

As for items, the better mages have 2 sets of items, just like they have 2 sets of armor. Hp and mp armor goes with hp and mp gear. A good mage should have a set of at least +6/10 hp and mp armor along with either uniques or +9 items. The best uniques include Rings of Magic, crystal rings, crystal belts, crystal amulets, mage earrings, lycaon pendants, and other items that give defense, magic power, or hp bonuses.

That's about all there is to mages. There are just a few more little things that you learn as you go such as "a wizard staff +7/56 won't hit for as much as an elixer staff +6/48" or "the level 45 aoe spells actually hit for the same amount or less than the level 33 aoe spells. They just have a much greater range". Lastly, try to get any scrolls that you need for your mages. People may look at a fire blast scroll for well over 20m or an impact scroll for almost 40m and decide not to get them. They really are worth it!