Lux is a support player, so if you not going only for kills, but helping your team win a match - this is champion for you. She can slow/stun enemies helping your team mates to pick up a kill :)

I think Ability power helps a lot a specially on a beginning of the game. Lux is mana dependant, that why mana regen is quite essential for her...

On Twisted treeline better to buy morellos than a RoA. It gives you cdr and ap. Im starting with doran ring, then boots of speed to complete Ionians boots as fast as possible, coz you weel get mobility and cdr. Than R Hat - to boost your ap and complete Lich B to get one more powerful shot. Im not buying any armour and mrs, because Lux is master of CC so you can keep a distance illuminating bushes with your E to avoid ganks.

Ill take clarity because lux needs mana specially spamming spells having morellos and Ionians.
Flash.... well its really good escape mechanizm and can help you to be in right place at the right time :) flashing through the walls.

Try to be behind your team mates, because you are VERY squishy... supporting them stunning and slowing opponent - sometimes pick up a kill. People often calling Lux kill stealer a specially with her ulti, but in my opinion there is no such thing like ks in team play.

This is my first guide. I show the way i play this champ. Hope that will help someone. Sorry for my english, but its not my mothers language ;)