Hi! I am Blueexorcistfire and this is my very first guide!! I am not yet done this guide yet so before you downvote or upvote please remember that this guide is incomplete and I will definitely be making some changes. If you have anything to say, please feel free to express your opinion on the comments below. This is how I play and this is my opinion.
And remember... THIS BUILD IS NOT DONE!!!

Lulu is a very unique champion with many slows such as Glitterlance and many interruptions such as Whimsy and an awesome partner, Pix. She has amazing survivability and can really help support! Although she is considered a support, I do not believe supporting the entire game can reach her full potential, therefore at the start of the game I play as a support, around mid-late game, I try to stay somewhat of a support, but I definitely start playing more aggressively.

Pros:
+Lulu is an amazing support
+She has so many slows
+She has a polymorph spell: Whimsy
+An awesome ultimate for a Cho'gath who just isn't big enough
+She has amazing survivability
+She is LOTS of fun to play as
+She can be more than just a support

Cons:
-She is squishy
-She is pretty darn slow without her boots
-To play at her best potential you will need practice using combos especially with Glitterlance
-Totally CANNOT win alone at the beginning

For Runes, I build Lulu offensively. I believe this well really help throughout the entire game. Magic penetration in her Greater mark of Insight, Mana regeneration from her Greater Seal of Clarity, and some AP from her Greater Glyph of Force and Greater Quintessence of Potency.

For Masteries, I pick 9 for Offense for a bit of AP and a pinch of CDR. I also gave her 21 Utility mastery points to fulfill her support role. Note that these decisions are not final, as I am trying out new masteries each time.