This is a generic DPS build that works with just about every DPS champions there is

The first item you are gonna get is the basic boots, these are great for getting first blood or running away to avoid a first blood.

Your second item will depend on how well you lane and the enemy team
1. you have problems with health get a vampiric scepter
2. if their are not stacking armor get a BF sword
3. if they are stacking armor get a pickaxe

Your third item will depend on you second item
1. if you got a vampiric scepter get a pickaxe or BF sword depending on their team
2. finish the clever if you got the BF sword
3. finish last whisper if you got the pickaxe

Your forth item will be situational boots
Berserker's greaves if you want to backdoor
Ionian boots if want cooldown reduction
Ninja Tabi if there are alot of auto attk champions
Mercury treads if there are alot of aps or cc

Your fifth item will depend on your champion's play style
Phantom Dancers if you need a sustained dps
Ghostblade if you need more dmg and armor pen
Trinity Force if you spam skills then attk

Your sixth item will start to build on your infinity edge, like always start with the BF sword then the Cloack

Your seventh item will start to build on phantom dancer to increase the crit chance and overall dps, start with brawler's gloves for the crit

Your last item will be the bloodthirster, the game shouldnt even last this long but if it is start farming it and try to stay alive, with a fully farmed bloodthirster you will crit around 900dmg on a squishy(around 2/3hit ko) and 400dmg on a tank(which you shouldnt really target unless you have to)

If the game still havent ended buy all the elixers and if you have any left over gold sell you boots for a bloodthirster or phantom dancer

I usually go with exhaust and ghost, with the exhaust mastery the enemy will lose 10armor and will be slowed for an extra 0.5 seconds which is great for chasing or running away, likewise for ghost use it for running or chasing

Other recommended spells are
Cleanse- you are a dps a blind/exhaust/stun will get you killed use cleanse to get of a situation like this

Flash- this a good alternative to ghost but this is more for running away(wall hacking) than chasing a champion

Ignite- with the new nerf on lifesteal and spellvamp this can be used for shutting down other champions that relies on them to stay alive, this is also useful against champions with high health regen and getting firstblood

Teleport- get this if you want to backdoor

Smite- if you are going to jungle

Heal- helps you avoid getting killed