I know a lot of people have already given some great advice, but I'll give my two cents anyways.
I've been messing around with his builds since he came out. Still undecided on whether Courage of the Colossus or Windspeakers is better, and pros seem similarly divided. I'd say both are pretty solid on him. Lately I have been building Coin to start (just gives a lot of money and mana, very strong on most supports right now) then (Edit: Sightstone first! How could I forget?) Ardent Censor, Redemption, Locket; pretty standard support build atm. You can build him tanky, but since he can get out fights pretty easily, I find the support items better generally.
His lane phase is actually pretty weak. You don't have much poke and your engage doesn't get crazy until 6. A Rakan lane is great at counter engaging against someone like Nautilus or Thresh, and he is very helpful when the jungler ganks. His strength comes from his long range engage when your team starts grouping.
Typical Rakan rotation is:hang back and let an ally walk forward. Use E to them, use R and W in in quick succession. Using R first will charm anyone on the way in, and guarantee the knock up. Then E back out. This is all done in a few seconds. Hopefully your team was already grouped and ready to follow up on your engage. If not, probably need to retreat. If so, then use your W and shields as needed, either to chase or to peel for your carries, whatever is needed. Don't forget to use your active items like Redemption.
Be sure your team is ready for any engage. Rakan is also good at following up engages, like a Malphite ult or Gnar ult. Don't be afraid to use your ult for peel too; it has a pretty short cool down.
Hope that helps! Glad you are seeing some improvements. Rakan takes some practice, but he's really fun and very rewarding to play.