Generally, as a support, you want to begin the game with your GP10 (Ancient Coin: Passive; Relic Shield: Tank; Spellthief's Edge: Mage), Trinket (Always Warding Totem until you have your Sightstone at which point you can replace it with Sweeping Lens), and 4 Health Potions (Which are upgraded into Biscuits with your Support Mastery Pages).

Mage Support: Your core items are Sorcerer's Shoes (Distortion Enchantment), Ruby Sightstone, Mikael's Cruicble, and Zhonya's Hourglass. Zhonya's Hourglass and Mikael's Crucible are purchased depending on whether your AD Carry is being focused (Mikael's), or you are engaging on the enemy team (Zhonya's).