I'm Virtual and i'm a 1900~ AP mid player. This is my first guide so constructive criticism is always welcome.
I've played about 10 games of diana and i think that diana is a really strong mid champion. Diana really fits my playstyle since she is so good in shoving the mid-lane and roaming.

For my marks i use Magic penetration because it increases your damage and there are no good alternatives for marks.

For my seals i use Armor or Flat Mana regeneration The armor seals will decrease the damage of minions and enemy autoattacks, which is very usefull in lane. In an Easy lane i might take flat mana regeneration seals/color] because it allows you to push your lane and harass more.

For Glyphs i use Flat magic resist. Flat magic resist glyphs help you farm without getting bursted down. They will also help you 1v1 the enemy mid laner.

For Quintessences i take Flat ability power . Flat ability power quints will increase your early game damage.

For masteries i take either 21/0/9 or 21/9/0
I usually take 21/0/9 because Diana is quite dependant on blue buff. I use 21/9/0 against really hard lanes like annie or another strong burst mage.