During Early game :

You need to play really agressive as a Miss Fortune player : during the firsts minuts of the game, she deals tons of damages with her Q. Indeed, if you are good enough (something that is not really complicated) you can touch the ennemy range or his support in each Q launch in minions on directly on them by rebound your bullet from one to the other ! It's really powerful. And don't forget your W that gave you attack speed and reduce by 50% heals done on the ennemy touch by your bullets during the spell activation (great way to counter his summoner spell "heal" !) ! Finally your E is only used to check bushs, to slow chasing ennemies and to maybe slow when your chasing (but the slow isn't that hard).
However, do not push your lane voluntary ! You'll let the ennemy lane take minions and gold and exp... and expose you to ennemy ganks ! Then be safe in any situation and don't forget wards which are the most important thing in LoL.


During Mid game (from lv 6) & late game :

Build you tons of damage by buying an Infinity Edge as soon as possible ! However, critical strikes are hazardous that's why you have to stay in the position of an ad range every time, never go before your allies, never go alone...
From here, you're a classical AD range, use your Q just after an auto-attack to deal twice more damages, use your W to gain attack speed and block the ennemy heals, and E to slow ennemies as before !
Furthermore, your ultimate on R can be use as a long range nability to steal dragon if you can't fight at, to end ennemies who're low and who're just too far from your basic range. It can also be use to engage a teamfight when the ennemy team starts to come on your, you'll deal to all of them 500-700damages something which's not negligible !

Enjoy MF, she's just a loose girl who's powerful, and this is only good ! :D