This Lux guide is very straight forward, I fancy my self to be a well above average Lux player and this will help you work some lazer magic.

Pros:

You can safely pick Lux first in draft play.
High damage output from range.
Full of utility abilities.

Cons:

Very soft champ.
Relies on landing skill shots.
Advanced positioning skills needed to be the best.

All four of Lux's abilities and her passie are very useful and effective when in a 1 v 1 or a 5 v 5 fight.
Passive:

Procs damage when you auto attack after landing an ability on any enemy. Very strong in lane if used correctly to poke.

Q:

Allows you to keep enemy in place long enough to land full combo.
Also allows you to snare enemies that are pursuing you.

W:

It's a shield that is applied to you and allies twice, basic but effective.

E:

Your main damage ability early on.
Also allows you to run away or catch up to champs duo to its slowing effect.

R:

Giant lazer that leaves nothing in it's path.
Land q, toss e on top of champ, land R, pop e, aa. Boom goes the dynamite.

Early farming can be somewhat difficult with Lux because you have to balance your abilities between poke on the opposing champ and procing your passive for minion last hits. Once you have a decent amount of ability power farming becomes very easy when you can toss out singularity and kill the caster minions instantly. UTILIZE your passive to last hit minions, your CS will significantly jump when you figure this out.

With Lux this may be the most important category, if you are positioned correctly you can nuke the opposing adc while tanding behind your team and kiting for your adc. Kiting the opposing team for your carry is very important for your teams health. Slow, snare, and shield your way through the fight and u will be surprised how many kill you can pick up.

If you have to pick your mid before the other team picks theirs, Lux is always a safe but very powerful pick. To be effective you have to be able to land your skill shots every time you are given the chance. Practice your positioning skills in team fights before you jump into ranked, her shield can be the difference in a good or bad ace.