Trinkets are always something I forget no matter the role xD
Btw, first of all see if you are more proficent as a tanky supp or a squishy supp. I mostly go Tank because I rarely see team mates go tanky on ANY char and because I've always played Paladinish characters in RPGs so I'm used to deal with being the meat shield.
Remeber that your role is to build up kills and numeric advantage, always be the link between the 2 spreaded groups (unless 1-2 ppl are just going suicidal).
Then remeber that your buildpath is the most dependant on what your enemies are. Learn to make use of your skills with the base values because it's really likely that you won't be stacking AP anytime soon.
Edit: while sightstone is something you get early it's not always the first item to rush, adapt to your char, your adc and then enemies.