Lux's masteries are the typical 9/0/21 ratio. But of course you can customize them to your preference. For example the one point you put in Summoners Wrath could be moved somewhere else if you don't use those spells. You could put another point in utility/Intelligence if you wanted to. Like I like to say (:P) It's your choice how you want to play Lux, the lady of luminosity.

Basically if you don't understand the item order it goes like this: First I buy boots of speed and 3 mana potions. Next I get sorcerers shoes and start building into mejajs soul stealer. Once I have it I get a rod of ages and start building a rabadons death cap. I keep buying mana potions because Lux runs out of mana quickly. Although after buying the death cap you shouldn't need to anymore. Next I get morellos evil tome for cooldown reduction. Finally I build an abyssal sceptar, but usually I don't ever get to because the game has ended ._.

Playing Lux you can use a lot of spells quite effectively. In my opinion and the way I play her I use teleport and flash. Flash is pretty much essential, don't ask ;D. But the first spell could be quite a few other variables. Clairovence and ignite are the other two that I will sometimes use. But of course it's your choice how you choose to play Lux, the lady of luminosity.