Start of with Hymn of Valor. Store stacks in Power Chord while traveling from base. Usually I get Aria of Perseverance at Level 2 and 3 especially is your ADR is getting harassed a lot or didn't get Doran's Blade. If you're doing well at bot, you can go get Song of Celerity at Level 3 then max Hymn of Valor.

Start of with 2 Green Wards, 1 Pink Ward, Faerie Charm and 2 pots. Buy Philosopher's Stone asap so you can start earning more gold, and don't forget to take a ward or 2 as well. Save up for Sightstone and get a pink or more green wards. You can get Kage's Lucky Pick if you want more income and the added AP helps too, especially if your ADR is doing well.

For boots usually plain boots is fine early one, but eventually you can choose between Ionian or Swiftness, after you got Ruby Sightstone and you're GP5s. Most people prefer Ionian but as support you get focused a lot, Swiftness plus you're E (and Reverie) means you could move around a lot. Chances are the enemy team will be trying to chance you buying your team time to damage them. In fact with all that mobility you can jump from bush to bush and confused most enemy teams. With your auras and item auras, just being there running around helps.

Most of the time, during mid game my items are just Ruby, boots, Philosopher's, a bunch of greens and pinks. Some games I even end with just those. As support you don't get the luxury of reaching those expensive items with your meager gold income and warding expense, but that's support life right?

For support items, it mostly depends on your gameplay, teammates and enemies.

-Aegis is costly but is very rewarding most of the time. I have experience a lot of game turning moments merely cause I was able to build Aegis. Upgrade to Locket when you can.
-Zeke's is great for your ADR but its not advisable to get it before Aegis.
-Reverie builds from Philosophers and is awesome in chasing or escaping.
-Pop True Ice on your initiator is great in controlling clashes. Works great with allies like Malphite, Amumu, Jarvan, etc.

I usually get Exhaust and Flash. Sometimes Heal works too especially in early game. Most people don't expect heal in high level games that the burst heal is deceptive especially when the two ADR are slugging it out.