Welcome to my first build, with this build you will support your teammates incredibly well and have a lot of fun while doing that. I hope you like it.

Lore:
Born to the prestigious Crownguards, the paragon family of Demacian service, Luxanna was destined for greatness. She grew up as the family’s only daughter, and she immediately took to the advanced education and lavish parties required of families as high profile as the Crownguards. As Lux matured, it became clear that she was extraordinarily gifted. She could play tricks that made people believe they had seen things that did not actually exist. She could also hide in plain sight. Somehow, she was able to reverse engineer arcane magical spells after seeing them cast only once. She was hailed as a prodigy, drawing the affections of the Demacian government, military, and citizens alike.

As one of the youngest women to be tested by the College of Magic, she was discovered to possess a unique command over the powers of light. The young Lux viewed this as a great gift, something for her to embrace and use in the name of good. Realizing her unique skills, the Demacian military recruited and trained her in covert operations. She quickly became renowned for her daring achievements; the most dangerous of which found her deep in the chambers of the Noxian High Command. She extracted valuable inside information about the Noxus-Ionian conflict, earning her great favor with Demacians and Ionians alike. However, reconnaissance and surveillance was not for her. A light of her people, Lux's true calling was the League of Legends, where she could follow in her brother’s footsteps and unleash her gifts as an inspiration for all of Demacia.

“Her guiding light makes enemies weary, but they should worry most when the light fades.” – Garen, The Might of Demacia


Runes:
Marks: The ap will help you in end game, you can also use cd reduction to cast your abilities earlier.

Seals: I use cd reduction, because you will need it in end game.

Glyphs: 10ap at the beginning of the game is very good + the 10ap from your Quintessences is imba!

Items:
Doran's Ring: I take this item very often at the beginning of a game, because it gives you the necessary ap and manaregneration, to keep a good lane.

Boots of Mobility: This item is very good for supporter, because you have to stay in your group.

Will of the Ancients: This is your first supportitem, it gives you +70ap,+20%spellvamp and your team +30ap,20% spellvamp, it is unbelievable how much it helps your team in teamfights.

Soul Shroud: Your second supportitem is one with 15%cd reduction and 12 manaregneration all 5seconds, btw. it gives you +520life(good to survive). The manaregnaration is nice in midgame, when your partner is often out of mana, but the cd reduction is much more important for the mid/end game, when your team casts the spells 15% faster than the other team.

Aegis of the Legion: It's for survivability, +12armor, +15magic resistance and +8damage for allies isn't bad and don't forget the bonus for your own.

Abyssal Scepter: +70ap,+57magic resistance, reduces magic resistance by enemy Champions in near by 20. Very good item for Survivability+more ap and to weak your opponents.

Last Item: You can choose a Void Staff or Zhonya's to get more absorb for your Shield.

Summoner Spells:
Clarity: You will need this spell, while casting your 'w' skill as much as you can.
Flash: Very good spell, you can escape from ganks or combine it with your 'q' skill to kill an enemy, who would escape.

Abilities:
Illumination(Passive): Lux's damaging spells charge the target with energy for 6 seconds. Lux's next attack ignites the energy, dealing 20-190 (depending Lux's level) magic damage to the target.

Light Binding: very good to bind an opponent, wich your team can kill then.
Cooldown: 16/15/14/13/12
Cost: 60/75/90/105/120
Range: 1000

Prismatic Barrier: this ability is your mainability, while supporting your team. It saves normally more than 20lifes in one game.
Cooldown: 14/13/12/11/10
Cost: 75
Range: 1000

Lucent Singularity: very good ability to harass your opponent, slow it or/and kill it.

Finales Funkeln: with this ability you can kill your opponent or prepare a kill.
Cooldown: 80/60/40
Cost: 100/150/200
Range: 3000

Early game: The best partner is a nice dps champion like master yi, after you reach lvl 4 or lvl 7 you can cast the 'w' spell on him and he can start the fight. Try to lasthit the minions as much as you can to support your team early with your item's.

Mid game: When you've got your boots, will of ancients and soul shroud, you will help your team a lot with it in teamfights and you will normally win the fights. Try not to die, stay behind your team that no one can focus you, cast 'w' as often as you can and don't forget the other spells, when 'w' is on cd, because they are very good, too.

Late game: your team will love you, try not to die, always stay behind your team, cast your 'w' skill through your team and kill your enemy.