Support requirements?
You can play almost any champion as a support, but there are a few things that are important for support champions to have:
Peel. This is one of the most important traits for a support champion to have (although supports with poor peel can still do well). Most types of crowd control are good for peeling, especially knockbacks and knockups. Some good examples of very strong peel supports are Alistar and Janna.
Sustain. This can be in the form of shields or heals, as long as they can target allied champions. This will allow your lane partner to stay in lane longer. An example of an excellent sustain support is Soraka.
Utility. This is extremely dependent on the specific support, and can mean a lot of things. This encompasses things like movement speed buffs, attack speed buffs/slows, and so on. There are many high utility supports, such as Sona, Nami, and Thresh.
Poke. A support with strong poke is able to instil fear within the enemies and force the enemy to trade CS (and thus gold) for damage. Good poke supports include Sona, Thresh, and Lulu.
Zoning. This is less applicable to passive supports like Soraka, but zoning is an important trait for support champions. The ability to scare off lane opponents denies them gold and experience, making it easier for your lane partner. Good zoners include Blitzcrank, Thresh, Taric, and Leona.
High base stats/damage. Champions with high base stats will have an easier time in lane, and champions with high base damage can support and still deal relevant damage without items. Examples of this include Zyra, Fiddlesticks, and Jayce.
In addition to the traits of the champion you select, you must also be mindful of your own characteristics. There are a few things you should do to be a good support:
Do not take farm from your lane partner. They usually need it more than you. The exception is when they certainly cannot get the farm (e.g. they are in base) — in that case, it's better that you have the gold than nobody.
Be careful about pushing the lane. When you push up, you render yourself more prone to ganks. Of course, as the duo lane, if you've been warding well, you can do this with slightly more safety than other lanes — but just because you can do something doesn't always mean you should.
Stick to your carry. Do not chase and ditch your partner. Exception: if your carry will die for sure, just run. If, however, you can sacrifice yourself to save your partner, strongly consider it; it's often much better to have a carry alive and a support dead than the other way around.
Ward efficiently. This doesn't necessarily mean ward everywhere; just be sure to ward the river, any gimmicky spots (for instance, playing support against a Purple team with a jungle J4 means you should ward the river brush), and the lane brushes. Do not be afraid to purchase and use Vision Wards to make sure the enemy does not have vision of gank paths, or if you're a zoning support, the brush you should be camping in.
Be very map aware. Know whether any enemies are missing, and don't hesitate to call mia/ss for other lanes. Ping your allies if you notice enemies ganking them, since they may not be looking at the minimap.
Think about your purchases. Aside from the standard Ruby Sightstone that every support should build, your items should depend heavily on the game. Don't just blindly follow a guide; put some thought into what your team needs. Strong items for almost any support include Aegis of the Legion, Locket of the Iron Solari, and Shurelya's Reverie. You should also try to always carry Vision Wards for vision control.