Miss Fortune isn't that kind of ADC that need lots of attack speed. She needs armor pen for her Q and R.
Miss Fortune deals lots of damage per bullet with her R and with lots of armor pen you can reduce a tank's health to half (experienced by myself)
She also deals magic damage with W, E and R.

Start with boots and 2 healt pots and 2 pana pots

Buy Tear of the Goddess

why Tear of the Goddess? Miss Frotune is really mana hungry, especially if you're spaming abilities like me. + At the end, Muramana gives you more AD than Infinity Edge and 1000 mana

Continue with Bloodthirster

After this buy Last Whisper for armor pen.

You will need some attack speed and crit, so buy Statik Shiv.

Wy Statikk Shiv?It gives you enough crit + the very useful passive in combination with Q

After this buy Black Cleaver

!!Start with The Brutalizer!!

Now it's time to finish Manamune. When the stacks are full, you will have a nice AD bonus.

Replace boots with Trinity for more damage, more Attack Speed, bigger movement

IMPORTANT NOTES ABOUT THE BUILD

If you play against a counter, skip tear of the Goddess and rush to Bloodthirster.

!!If your counter is not that good and their support doesn't do the right thing, you can still buy Tear of the Goddess!!

If you skipped Tear of the Goddes, make the build in the same order, but last item should be Infinity Edge or Zhonya

!!Why Zhonya? Because it gives you a nice Active, bonus armor, and lots of AP which is good for your Q and R in combination with AD and for W and E wich are AP only abilities!!


*ATENTION*
If you need extra damage right away after Last Whisper, but you need some attack speed too, buy Zeal and The Brutalizer and finish Statikk Shiv and Black Cleaver in the order you need.
Replacing the boots with Trinty is very important. Replace them when you have enough money to buy full Trinity, not pieces.

It's important to start with Q then E then again Q and after this W
Why? Q has lots of damage against champs and E in combination with Q it's really good (Slow+ Magic Damage+ Physical Damage). Put 1 point in Q at lvl 3 and at lvl 4 1 point in W.

The R deals damage per bullet and with all the armor pen this build give you, you can kill a tank more easy than before. Squishy champs can be killed with 2-3 bullets (experienced by myself). For them you can use just E - Q - Auto Attack

Try to learn how to use Q properly. If you do this, you can make a double kill against 2 low hp targets OR you can use it to go in a minion then to a low hp champ (experienced by myself)

W is really good used against turrets. While dealing damage to a turret activate your W for extra damage (it's not a lot without AP, but trust me, it's still really important.)

Be careful at lower levels with the spells!!
It's better to last hit minions than using your abilities.
After you have Tear of the Goddess you can use your abilities on minions more, but be carefull to have enough mana !!Try to never go under half while farming!!