I'm building my first rune page for Katarina and was wondering how I should go about doing this. There's many options that it's kinda confusing. Right now I have 3 ap quints. Other than that I'm kinda clueless. Or, if it would be better for me to just build AP runes not meant for any single champion how should I do that? thanks for any help

Standard AP Mid stuff:
Magic Pen Marks (these absolutely are not negotiable)
Flat Armor/HP per level Seals
there are really no good offensive seals, these are much more efficient, armor seals are fantastic in lane and health per level seals are good in teamfights. I prefer flat armor seals in general because they're more general. For champs other than Kat, flat armor is better, also you NEED them if you're going up against AD mid lane like Talon or Pantheon.
AP/AP per level/flat MR blues
I like flat MR due to winning trades lane phase (and I like being tanky). Anyway, any of those are fine.

That's your rundown.

Mpen reds
Flat Armor yellows
MR blues
Move Speed Quints

I agree with Amoc except personally on seals I like Hp/Lvl.

Mpen Reds
Hp/Lvl Yellows
Magic Resist Blues
Move Speed Quints

sure i guess. The yellows barely even matter. Hell go with GP5. You can go with no yellows for how much they actually help.

I run:

Mage Pen reds (+8.5) Cause we like to do more dmg
Armor Yellows (+13) Reduces minion/auto attack harass
AP/Lvl Blues (+28) More ap is better lategame than early
Swiftness Quints (+4.5%) For chasing and escaping ganks/ect...

Flat MR is way better on Kat I think for blues. Her early game now is totally suck so the MR is really important. The way she builds makes her a total glass cannon. She ends up with ~400AP after her first few items and and by then 28 AP isn't going to be all that important and those runes are next to useless early game as well.