Ambush - Twitch hides in the shadows, waiting for a nearby enemy. If he attacks, he will gain bonus attack speed.
After 1.25 seconds, Twitch becomes stealthed for 10/20/30/40/50 seconds. If Twitch attacks a unit while Stealthed, he gains 30/40/50/60/70% attack speed for up to 10 seconds, depending on how long he was stealthed before attacking.

Cost
60/60/60/60/60 Mana
Range
20

Debilitating Poison - Infects all nearby champions with Debilitating Poison, slowing their movement speed.
Slows nearby enemy champions' move speed by 30%, plus 6% per Deadly Venom they have.
2/2.6/3.2/3.8/4.4 second duration.

Cost
80/80/80/80/80 Mana
Range
1200

Expunge - All nearby enemies secrete Twitch's toxic venoms from their bodies, dealing damage for each stack.
Deals 30/60/90/120/150 (+1) magic damage to each nearby poisoned enemy, plus 20/30/40/50/60 additional damage per stack of Deadly Venom applied to them.

Cost
90/90/90/90/90 Mana
Range
1200

Spray and Pray - Twitch closes his eyes and fires his crossbow, spraying powerful piercing arrows ahead of him.
Twitch's attacks become long range, passing through units, and he gains 15/25/35 Attack Damage.

Lasts 12 seconds or 5/6/7 attacks.

Cost
150/150/150 Mana
Range
1200

Deadly Venom - Twitch's attacks infect the target with Deadly Venom, which deals damage each second, stacks up to 6 times, and lasts up to 6 seconds.

I play aggressive using 1-2 hits + expunge, because I have the advantage of life steal and heal spell. By lvl 4 you should have enough money to get your boots and try a kill. Sneak behind the enemy and stand exactly on him. It's harder to target you and harder to run away. Most players will try to run as an instinct since you are so close and have the passive advantage (attack speed). As a mid player, after the 1st kill i put down a vision ward in mid and I teleport behind the enemy to score a second kill. I never leave mid until lvl 9-10 because i want to farm my zeal.
Early game is better to get some attack speed instead of attack damage, because everybody is squishie and you do much more harm with attack speed (more poison stacks). It's better for farming too, since Twitch needs a lot of gold to make this build.

In mid game you should have your 1st Bloodthirster. This is when twich is dangerous. The damage is enormous considering the health of the enemies. Just remember you will be squishie from lvl 1 to lvl 18, so don't be a hero.
In team fights i always position myself behind the enemy lines and use my ulti. This way you can get the kills if someone is trying to escape. Nobody will flee the opposite way (where your entire team will be). Just pop up a little later, when all their abilities are on cooldown.
After bloodthirster, get your Black Cleaver.
Don't even think about getting survivability items, you will be squishie anyway. Black Cleaver applies on hit and it's great with your ultimate. With your attack speed and ultimate you can kill the entire enemy team if they fail to focus you in time (using expunge).
Don't follow enemies for kills, this means they get the cooldowns and stand a chance against you even with low hp.

You should finish Phantom Dancer and Infinity Edge. Twitch is one of the few champions that can get all items even if the game lasts only 40 minutes.
By now you should have enough damage to fight anyone. You see something moving on the map, it's yours. Nobody can survive your ulti+expunge so be confident and attack at will.
If more than 2 enemies, get a partner to assist. Try to focus the one with oracle, in case you decide you want to flee and use stealth.
In team fights you should pop up from behind your allies, be untouchable. By the time they get to you is just to see you closer when they die.
I usually finish 30/2/2 or something. I get few assists since I am the one who gets the kills.

My main advice for Twitch (or any carry) is simple: Try not to die. The best games you will ever play is when u die less.
Try it, try and see how much u can survive, how many minutes. If 15 minutes have passed and you did not die with Twitch, you are already OP. You want that, don't you? It's doable, just try it.

Pros: High Burst Damage, a lot of kills, stealth, fun.
Cons: Squishie, Bad in the wrong hands, useless without a good team to defend you.

- Don't follow people for a kill. Some champions can kill you in 2 shots, so better get him next time.
- Don't try to save a fellow allie if he's attacked by the whole enemy team. It's better to wait behind the turret and use your ulti (kill all minions and maybe 1-2 champions)
- Don't stay too much on the same lane if everybody on the map is missing. They will try to gank you, for sure.
- Don't buy tiamats, they are useless. You don't need tiamat.
- Don't buy health items. You are squishie and that's that. If you don't like it, buy another champion. All u need is attack speed, crit and damage.

- Always get a vision ward on your lane to see if they have wards.
- Late game you should always get oracle to see wards and avoid/destroy them.
- Always scout for baron, red/blue buffs.
- Scout for enemy teams waiting in bush. I sometimes get penta-kill like this.
- If you missed a team fight, position yourself behind enemy lines to catch the fleeing enemy.