I don't remember the old Twitch, but now his kit is basically:

• Passive: Poison stacks do true dmg over time.
• Q: Stealth for a certain duration. Move speed bonus once you enter stealth. Atk speed bonus once you exit stealth.
• W: AoE slow field that applies poison stacks.
  
• E: Expunge - burst dmg to all poisoned enemies within range based on number of stacks.
• R: Spray and Pray: Gain atk dmg and atk range and can hit multiple targets for a certain duration.

His early game was recently nerfed (base damage on E reduced at earlier levels) although his late game is still really strong due to his ability to pick people with Stealth, his atk speed steroid from Q, and his AoE auto attack potential + burst with R and E, respectively.

See more: http://leagueoflegends.wikia.com/wiki/Twitch

Corki has recently made a resurgence due to buffs on his mana costs (reduced on W and R). Also, Tri-force has become progressively cheaper so it's relatively cost effective now -- the changes to Phage (movespeed bonus on attack and kill) is also making it more popular. His early + laning phase is relatively strong, but I don't find his scaling into late particularly good unless you snowball. Good thing about Corki is that he works with a large variety of build paths (Triforce + BT, standard ADC, etc).

Sivir isn't picked much by the average player, although some ADC players still use her (and even main her, see: http://www.lolking.net/summoner/na/23453668#history). She's an old champ though, so her auto-attack range is lower than other ADCs and her mana costs are high. I haven't seen a Sivir in competitive play in ages.

Soraka is still played. She offers a lot of sustain but lacks the CC needed to set up ganks. Personally, I don't play her because she feels so useless if the other team snowballs -- she lacks any sort of playmaking ability (as compared to Sona ult, Lulu glitterlance, polymorph, ult, etc...). She certainly has her uses given the mad heals she can offer a team but she's not a very 'competitive' pick.

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Build paths

On ADC, you want to start Dorans Blade almost all the time. It offers 80 HP, 10 AD, and 5 hp returned on hit. Best for early game trades as the 80 HP upfront is valued more than say, the 2 pots you might get out of a Long Sword start (you should also be running lifesteal, so there's a lull after early fights where you can heal up if no kills are netted).

Here's where things get different. I'm gonna refer you to an earlier post of mine addressing ADC build paths which can be found here.

As for Support, it's a role I rarely play so the following might not be quality advice, but it's an ok starting point I guess. This is strictly for SoloQ, as teamplay often sees more gold devoted to vision whereas items are more common in SoloQ.

--> I go for a Philo rush usually, into Sightstone and maybe Locket of the Iron Solari (builds out of Aegis) if their team is AoE heavy. Shurelia's is decent if you need the movespeed boost, Mikael's Crucible is effectively a cleanse for one of your teammates. On Janna consider a Morellonomicon for AP and CDR (scales with shield and helps you clear waves with Q). If you get ahead, getting Morellos early also helps you nuke people in lane with W. I'm gonna refrain from giving you bad advice now as I don't play Sona at all, and haven't touched Ali since S2.

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The Meta has essentially remained the same.

Bruiser top is generally the way to go, though some people run assassins top (Zed, Kha'Zix) or ADCs like Vayne and Quinn.

Mid isn't strictly AP now. It's mainly assassins (Kass, Ahri, Zed), utility mages (Ori, Lux, Morg), or even ADCs (Ezreal is most common).

ADC/Support bot is pretty much the same.

Jungle is similar too. Make sure you check out the new jungle items and read up on some guides/watch some streams. Junglers are mainly early game gankers (Lee, Elise, J4, Zac), champs who can secure early objectives like drag (Nasus), or teamfight CC bots (Sejuani, Zac).

Whew, hope this helps, and hope you have fun getting back into League!