For a support champ a general rule is that you build a sight stone. Thats really the only general rule. You should never lock into any definite build path on a support champ. Your role in game is broken into 3 parts. part 1 is the laning phase, and your role is to disrupt the enemy adc from acquiring cs and to provide your adc with the means to win lane. This does not mean to give kills to your adc, there is a lot of misconception about that. Though generally its more beneficial for you to allow the adc to final shot enemy champs. Also during the first part, you should look to gank mid lane, because you're the team support, you're not the adc support. In situations where you might get a double in bot, and you have enough health you should travel up to mid to see if you can get a kill there as well.
The role you play in game post laning phase, or part 2 is that you're a second jungler. You're still not farming the jungle, but you're roaming and giving sight to the map. making sure lanes dont get push into you too much, and being available for battle. you should keep areas which are common battle grounds warded and sweep for enemy wards. This gives your team the ability to coordinate plans and communicate better. during this time in the game there typically arent any major team fights. but you should try to stay close to one of your allies given than they encounter an enemy or you do. In either case you have enough cc as a support to stall the fight long enough for your ally to kill them.
The third phase of the game is the end phase. This is typically when all out team fights will happen. In this current meta I believe the role of a support is to provide engages, and cc during team fights to keep your adc or carry in general alive long enough to kill the enemy team. I like items such as face of the mountain, iron locket, and randuins omen for the added shields and cc they provide in fights which are sometimes enough to keep your team alive in battle. For morgonna specifically, I don't play her. But she has a massive stun. I would suggest in your build path to build a frozen heart for the 20% cdr. That also provides a great passive in battle for your team. But this is a recommendation, and you should build each game differently dependent on the enemy team composition.
I hope this helped, the role as a support is a confusing one, I know.
