In lane, Lulu's job is simple, make your carry's life easy and the enemy carry's life horrible. while you have high health and mana, use your spells stupid damage to cripple the enemy carry using the E > Q combo, E an enemy, which ****ing hurts, then Q them at point blank while pix is still floating right next to them for an easy shot, this also ****ing hurts. When your health is waning from the attacks the enemy will throw at you (so like, 50% hp and below) back off and let your mana and health regen while letting your carry take point. use your E and W on your carry so that they're extra tanky and fast, making them a super ADC. You can also fire your Q from your carry's position while your E is still affecting them, so dont forget to use this to poke and slow your enemy's from a safe distance.

during teamfights use your abilities to help whoever happens to be in danger at the time. There's really not much to it, just use your spells as often as possible to be as helpful as you can. Also dont forget to use your items active effects like the Twin Shadows to hunt down enemies and slow them so your team can crush them, or your mikaels Crucible to heal your team and cleanse CC off them in a pinch. Lulu has essentially 6 spells along with whatever active items you pick up. You dont do much damage as lulu late game, so you want as much utility as possible to help your team in other ways.