Stay back and poke with your q. Good combos is to w yourself, get in range to e them, so you go e>q>auto which procs thunderlords. Another good combo if you can't reach them is to e a minion and q from afar like that. One thing I love to do, which is usually unexpected, if you poke them enough they will stay back far but might get greedy to stay. If they do this you can cheese them by flash e>ignite>q(>auto):D People often underestimate Lulu damage.
In teamfights important things to do are to peel for your teammates! Land plenty of glitterlances on them, often it is more important to shield your teammates then to e enemies unless you can get good combos off. Keep your adc alive, Lulu is very good at this! Important thing to decide is if you need to ult your tank when they go in fights to knock up all enemies with it or save it for later when someone is low.

On The Easy Side: Champions like Vel'koz, Brand, Ahri, etc. Basically AP Casters you can have an easier time with poking them, using your e to shield from there poke and your ult to stop them from killing you.

Lulu is a very safe laner, making her easy to poke with and if behind, easy to not die with and just farm. However, these matchups are on the harder side-
Ryze, Zed, Veigar, YASUO Talon, Fizz, etc. - basically, beat them early or you will have to play passive. However play it right and they are not necessarily hard to beat early.

Strengths:
Poke- She is good at poking champions from afar and has a good combo to deal good damage.
Waveclear- her glitterlances are very good at clearing those waves
Squishies- With this build, a good combo specially when you are ahead will get you kills on squishies fairly easy.
Teamfight- Good at helping teammates, keeping them alive, kiting backwards because of q slow, catching up because of your w, and your knockup ult :D

Weaknesses:
Lategame- Will not do as much damage, has trouble killing people (keep in mind if there mid/adc is squishy you can still kill most likely)
Tanks- Does not do much damage vs tanks. Best you can do is to help slow and maybe w them and get your adc to kill them
Reliant- You are reliant on your teammates to do well if you reach late game. Since you don't do as much your more of a supportive mid laner, you can kill there squishies if you can get on them but other than that you are there to protect your adc/jngler/top w/e and help the team. Get those slows off and try to hit glitterlances on the enemy squishies. Problem is if your adc (who is main one to protect) doesn't position / gets caught out / is behind / etc. it makes your life hard.