If you're still leveling and learning to play, here's a set-up that will work on EVERY single caster (AP, supports, even some physical casters like Urgot):
Magic Pen Marks
Mana Regen/level Seals
CDR (flat are better, scaling are cheaper) Glyphs
If you're still figuring out which champions you like, the bonus of this rune page is that most of the runes will work on literally just about everyone (even AD champs can benefit from health, CDR, and mana).
If you're pretty confident that you want to play AP, then you should use this set-up instead- it's more specialized for AP mages:
Magic Pen marks
Mana Regen/level or AP/level Seals
Flat AP Quints
This'll have you doing tons of damage, but the lack of mana regen and health means it'll punish your mistakes in lane harder.