Be Cautious with this build. It makes you the team's main source of damage, but you're very squishy.
Don't rush into a team fight yourself. Determine whether it's safe to start fighting (Wait for Amumu's ult, Ap Caster's to use their spells, CC, etc).
Farming is a must. 250+ CS is recommended. Try to get 300 CS. Don't decide to farm if your team 'really' needs you.
Spam your missiles; they're stronger than you think. Save Valkyrie for last to chase or run from a team fight.
Try, try, TRY to get a solo lane, such as Mid, solo top (if another AD carry is top) or bot with a support.

Pros:
+Maximum Damage
+Superior to other AD Carries;Watch their health go down (unless they are more fed)
+Easily annihilate champions not Tanky
+Can sometimes 1v1 Tanky Dps


Cons:
-Pretty Squishy

Start with Boots and 3 HP Pots. Farm and harass until you're level 6. By the time you're level 6, you should be able to get at least 1000+ gold. Don't immediately go back to get items unless you have Teleport. If you don't, try to push with your Missiles and Phosphorus Bomb until your minions are at their turret or the enemy goes MIA or went to buy items.

Buy a Tear of the Goddess and (depending if you used them or not) 2 more HP pots. Go back and farm. Gank if you have the chance. Try to get 1115 gold. You can wait to get 1685 gold to buy Manamune and Berserker's Greaves.