From after playing 200 games, this is what i would do...
1. Get 10+ dmg sword at start and a potion for last hits. Dont auto attack unless minioons are going to get hit by towers. If the tower attacks minions, its hard to get last hits, therefore hard to farm.
2. Dont focus on killing tris, focus on last hitting, you can do the killing later.
3. The other tris would mostlikely get hp regen / lifesteal, while you have sword, so you can last hit alot better.
4. Never gank unless your team needs it and you are positive you can get a kill, otherwise, farm!
Main thing, out minion kill him/her!
For your items though, this is just a personal choice... i would...
1. Get beserkers boots then two zeals and a sword of the occult. Finish off with two phantom dancers and an infinity edge. Game usually ends after 1 phantom dancer, so i never decide wha tto get for the 6th item : ). Lifesteal is bad for rangers, because when your using lifesteal, that means you are stupidly engaging with the opponent and getting damaged. you are a ranger, let your tanks tank, and your ranged dps dps.
Another build i like is ...
2. Stack the black axe things (not sure what they are called), that reduce armor per hit. I would get beserkers boots and buy 5 of them, for the utmost dps sensation. doesnt sound very sensible at first, but trust me, try it.