Lux is a pretty simple champ with my build/runes. You play safe, closer to your side of the turret warding nearby bushes. Auto minions but avoid pushing too much. If you see the opportunity to unload a high amount of damage or even a safe poke, do it. Get those stacks of dark harvest! Keep last hitting, don't get turret plating unless it's open and uncontested. The most important thing is to not fall behind. Roaming down to bot at level 7 or 8 to assist or get kills will let you increase your ultimate hunter which is important. Buy the items and at level 16 you should become unstoppable and garuntee a win for your team no matter what.

Best of luck! practice makes perfect.