You and your ADC

Supporting is generally considered to be the role picked last. There are some people that love playing as a support and always plays as one, but it's common for people not to want to do it. Playing in bottom lane, you need to communicate properly with your partner. It's best to have a team of course, but not everyone has that option at their disposal.
Your job as a support is to ping for your lane, ward your lane, and defend your adc in laning phase. Your ADC is probably going to want you to help them get cs, not get cs yourself. Some ADC's don't even want you to hit minions. They are going to want you to ward and help them get gold as well as protect them in battles. It's a good idea to build things with active abilities that will help both you and your laning buddy survive.
Early game, I usually start off with a couple stealth wards, a couple health pots, and the coin for the talisman. This allows you to know where the enemies are in case of early ganks and now if they want to sneakily B in a bush as well as get gold slightly faster and heal yourself if needed.
Since I main Thresh, I usually get heal and flash as my summoner spells because he doesn't have a healing ability power, and flashing over walls can really help you out. If you build right, you can get away from most stuns.
A combination of flash, talisman, heal, and the box if needed, and you'll be just fine. Doing which first depends on the situation. Keep in mind, talisman can aid your friends to get away as well. Heal should be used first in this combo if you see your health dropping fast, such as in a 5 v 1, which is very hard to get out of.
Also, keep in mind that the ruby sight stone is a great idea to build. before that though still ward with stealth and vision wards. Don't just not ward! That's a big reason that you will get yelled at for.
Overall, if you get your adc kills, plenty of cs, lots of gold, and a nicely warded map, you will be doing excellent!