Well to not get flamed like i do everytime i speak,this is how i play and ''abuse'' supports to get elo i am not a god support but i consider my self not experienced so if you want to disagree feel free to do so.
Supports are really a role that varies alot,and i really mean ALOT you but this are the main 3 support goals for it to fufill and for a champion to be called a support.
-Kill
-Sustain
-Poke
and Kill>sustain
Poke>kill
Sustain>poke
And ill be calling this the support law from this sentence onwards
But you should never just be fixed to this as there are many different ways that a lane can be played so do not stick to this like a robot.This can help you only to a certain extend
Support tier list
Sona>All
Why you say, sona has the skills that all different categories of supports Which are kill(Q cord,Q buff and ult)Poke(All cords and q)Sustain(Heal and armor buff)Teamfight presence(Ult and all auras).With all these abilities, she would be the highest skill cap support,so if you want to play support,i wont recommend to try her 1st.
Items i would go are Farie Charm 3wards(with the gold mastery you can get) and 2pots, the when i base i would get my Philo stone and wards.I would then get items depending on my lane,the better the lane is the more GP10 i get (max 3).Support items i would go for Starks,Shyrelia,Soul shroud,Aegis,wota,spirit visage,oracles.