Twitch is a very squishy champ and should never initiate fights. You should try sandwhiching people in by going behind them and unloading and destroying them. Flash into a ambush can save your life if you get caught. Also Last hitting minions and building up money will help put you ahead and get your build done faster leading to more ganks.

Pros:
-Great carry, can annihilate teams in a matter of seconds.
-Is a great crippler
-Can find out where enemies are
-Can escape with Ambush(flash if needed)

Cons:
-Squishy, easily nuked down should never start team fights
-Easily CC'd
-Since you are a squishy little carry you are targeted first
-Oracle's and wards can stop you from scouting, and escaping with Ambush

Armor Penetration Runes- great for Twitch since he deals a lot of physical damage. Also, since he does so much damage, he can shut down virtually ANYONE!
Dodge Runes- Twitch is very squishy, and he is a prime target for ganks! Dodge runes increase his chance of survivability a lot.
Damage Runes- I believe as twitch your nuking people so damage runes will help you burst people down more quickly and initially help you get fed quicker.
Health Runes- Gives Twitch a little more survivability sense he squishy
Armor pen Runes- More ARP = More Damage.
The yellow runes are debatable! I feel safer with dodge runes, but you can use more damages runes instead to give you even more nuking power.

Ignite: If your looking for a quick kill early game ignite will help and with your poisons you cant go wrong.
Flash: This is a must cant stress it enough it will save you. Flashing through walls into a quick ambush and your homesafe.
Exhaust: I think this is a cripple summoner spell. You already got your slow from your ability but exhaust can help you win a 1v1 fight if needed so its up to you to use it.
Teleport- Sense i go for berserker greaves i like to use teleport so i can get as much mobility as if you had boots of mobility. You can catch people killing minions pop up own em and book it also very helpful to stop people from taking down your turrets.
Cleanse- This Summoner spell you can get if you like instead of teleport. If you dont run Flash/teleport then Flash/cleanse it the way to go

Dorans Blade- Amazing start off item gives you hp damage and lifesteal. End game you can sell it for a situational item if needed.
Berserker Greaves- I pick these over Boots of mobility because i want the attack speed and i feel comfortable enough with the movement speed 2 instead of 5 and with teleport i can get around the map almost as quick and can get off as much ganks.
Infinity Edge- Once you get this item is when you will start getting fed and doing insane damage
The Black cleaver- Gives you attack speed, Damage, and helps reduce armor.
Bloodthirsters- This is debatable but i like them personally because twitch is squishy and sense you are going to be hitting super fast why not do insane damage and heal yourself. Ive destroyed champs that usually would wreck twitch 1 on 1 almost instantly because i have so much lifesteal and hit so hard they cant do anything before they move.
Situation items- This is up to you depends how the game plays out. You can switch a bloodthirster with a Madreds Bloodrazor or even sell your dorans and get a banshies veil which will give you more survivability vs casters and such.

The items you pick are up to you i switch them around depending if i see a hp tank or if i see thornmail thrown into the situation which is where the madreds is a great item.

Laning phase- I think Getting expunge and going for a instant gank at lvl 1 is super easy especially if your partner is ready for it. Which is why i dont grab ambush till lvl 2. When laning last hit as much minions as possible and help out other lanes thats why you are a stealther. Mid is the most prone to ganking take advantage of it with your mid partner and get easy money and help shut down mids exp gain.

Watch Out for wards and oracles these can shut you down getting help from people to shut them the wards down or kill someones to get rid of the oracle can make them waste money to shut you down and make your team get ahead in money.

Mid Game- Laning phase is over and this is where spray and pray will help set up triple kills,quadras,and even pentas. People will also start breaking off from their team and be easy kills for you but dont always go for kills backdooring turrets is amazing as twitch and will help your team win.

Late Game- Now by this time your build should be about done or even done and you can just mow down people instantly and help your team push for the victory. Late game people will start getting oracles and wards to shut you down be aware of this and dont get out of position and ending up getting killed.