Necessity Build
Start with Doran's Blade then buy
1. Doran's Blade
2. Boots + Tear of the Goddess
3. B. F. Sword + Avarice Blade
4. Berserker's Greaves + Elixir of Fortitude
5. Infinity Edge
6. Vampiric Scepter (sell a doran's)
7. Statikk Shiv
That's your core.

FollowUPs

1. Bloodthirster / Blade of the Ruined King .
Do you wanna kill some high helthed target,are you afraid of thornmail,do the enemy's tenacity to cling to you frustrate you?Then build Blade of the Ruined King if not Bloodthirster will be this dude.
2. Are you going to spam-kyte to oblivion?Then build Manamune , if not then sell that Tear of the Goddess and get yourself a Last Whisper if not but you're using basic attacks more than you kyte then agains sell your Tear of the Goddess and get a Trinity Force .

Situation Calls

Absolute dominance is occult Sword of the Occult
Enemy won't die(heals like wolverine) : Executioner's Calling
Sassy summoners treat you like an outcast(cc you hard) pull of some sash yourself : Quicksilver Sash
(upgrade into mercurial Mercurial Scimitar endgame)


For Defence Build

First choice : Guardian Angel
Second choice : Banshee's Veil
Situationally : Iceborn Gauntlet , if and only if you need the extra kytability bad.

If you're in a pubstomp situation nevermind defense
If you're positioning and/or being protected properly 1 should do
If you're focused hard and provided you can at least kyte properly go for two

Special mention

If you can and/or will stand your ground,sell avarice in the 30's-40's mins and buy a Phantom Dancer
Buy some sight wards in the late 20's if not earlier(sell a Doran's Blade) if you're serious about winning.

This is too new for me and i don't think ill commit to even the simplest guides but still it bothers me and i have to apologize for not writing a proper guide as well as not knowing how to.Also i might update this should i learn how to actually implement the items in the stat calculator which makes the guide look like waste of space currently.Finally soz if i neglet to answer to any comments.