Note that there are (at least in the main meta) Two primary roles for supports. A Tanky Support, and a Poke-heavy Support. Currently it seems like the Meta now sides with a Tank based Support champions (like Blitz, Bard, Braum, Alistar, Thresh, Taric)
With a Tanky Champion you would obvious build Defensive items as well as Health. Usually getting the Relic Shield line of the support item, as well as SightStone (although this should be automatic for any type of person playing support)
Movement Speed Boosts (as Tanky supports are usually slow)
or at the very least Magic Resistance Boots (for that Tenacity)
Then for Defensive usually getting Locket of the iron Solari, Iceborn Gauntlet, Tornmail, Warmogs, Athene's Unholy Grail, can all help with Armor, Health, and MR.
For a Poke Heavy type of champion like Sona, Nami, Lulu, Soraka. They can help usually with their skills being long range to help keep your enemy from either attacking your ADC (or any teammate), getting or denying them CS.
With Champions like that youll want to buy items that help scale on your Abilities to harass your enemies the best you can, while also getting items that can buff up your teammates. Items like:
either of the beginning Support items Amulet Coin or SpellThief's Edge can help in the long run or either giving you more damage (STE) or helping you with mana regen to continue to use your spells (AC)
Again SightStone is a must for helping to Ward, but that Health boost is good as well.
The rest usually falls in line of helping you and the team with an Aura boost from the items like: Lich Bane (especially on Sona with her Passive can deal massive damage on an opponent) Rylai's Crystal Scepter can help by slowing your enemies plus a good amount of a health boost. Abyssal Scepter not only helps with Magic Resistance, but also negates the enemy MR while near them (in team fights) Zeke's Halbinger will work best especially if your ADC can pull off Crits. Along with Omewreker, ZZ'Rot Portal, Banner of Command can help you get objectives as they help disable Towers (Omewreker), helps improve minions (BoC), or can help summon a Voidling to help take down turrets (ZZ)
Boots are usually movement speed since you want to be fast to catch up to help your teammates where ever they are, or the Magic Pen/Resist can help as well
Also note with Supports youll want to get to 40% Cooldown fast so your skills are up as frequently as possibly. Majority of the items i list do help with CDR as well. But those are what i usually try and build (with a few exceptions of course) that helps me out with my games.