I can't believe I have to tell people this, but OCE, right?
As a support, one of the most basic things you need to do if you're versing a match-up that puts you under turret is manipulate minion HP so that your ADC can farm effectively.
Ranged Minion: 1 turret shot + one AA from support + one AA from ADC = 1 CS for ADC (turret shot and support AA can be in any order, but watch out for friendly minion aggro and adjust accordingly. Also, if you're, say, Thresh with fully charged Flay, it'll just take the minion. So the equation is not set in stone, it's just general.)
Melee Minion: 2 turret shots + one AA from ADC = 1 CS for ADC (but if the enemy melee minion is not at full HP, then HP manipulation will be needed)
Also, be mindful of doing random crap that does more harm that good. For example, as Karma, don't randomly Q ranged minion wave that is approaching turret when you're supporting, say, Vayne. Maybe an ADC like Caitlyn, yeah - but even then, probably not.
Like, not doing/doing this sort of stuff really gets on the ADC's nerves - and rightfully so if you ask me - and forces them to chat stuff to you rather than concentrate on the game.
Also, don't run around with a sightstone and a yellow trinket. No, holding five wards does NOT mean you have stronger potential vision control. It means you're running around being seen, unable to clear vision, unable to set up your jungler for a lane gank, unable to prevent your team's movements from being seen, unable to make sure you're moving through the enemy jungle undetected, et cetera.
Also, use Vision Wards. For the love of God, don't be the Support that buys less Vision Wards than his ADC. Don't be stingy with them FFS. You can just go back and buy some more. If you notice the enemy warding behind you and your red trinket is down - or you haven't changed to red after Sightstone bcause you're actually that bad - use a Vision Ward.