Janna is hard to make plays with. She's good at interrupting the enemy attacking and the plays they try to make.
The biggest problem I see with thresh is they always buy relic shield but always are really bad at using it. Consider the coin if you're not good at last hitting with very small damage, or left click on minions so you see their exact health and know when to hit. My biggest pet peeve as adc is when supports play a hook champion and only go for it like 3 times the entire laning phase. You're a play maker; don't always wait for them to come to you because it doesn't happen often.
Another support I like to play is braum. He is very defensive and works as a great counter to hook champs since you can jump and shield the adc when they're hooked, and he can make a good counter attack with passive and q slow.
If you need a mage support, Zyra is really good since she has high damage that you can win the lane by yourself because some times adcs just refuse to do anything.
To improve at the role, I'd suggest playing adc. Playing adc has made me understand what a support should do and made me a lot better at the role and vise versa.