Rakan is a pretty solid support due to the fact that he can airborne enemies, allowing the ADC or other teammates to attack, he has a decent shield that he can use twice for the cost of one, a skillshot that if hit will provide a small amount of healing for him and nearby teammates (this is arguably his worst ability because the healing is pretty bad and you need to reliably hit it to get any kind of reliable healing) and finally his ultimate deals a small amount of magic damage but has the potential to charm the entire enemy team if you can time it right.

Q- Gleaming Quill

I recommend unlocking this ability at Level 3 because it has a pretty hefty mana cost and is, in my opinion, his worst ability

W- Grand Entrance

Unlock this ability second, it is a great engage tool and if you have your E unlocked an a teammate is in its range, you can disengage really easily.

E- Battle Dance

I personally unlock his E first since if leashing your jungler you can shield them from the some of the damage they take from the buff. It also allows you to negate some of the early game poke from enemy ADCs and Supports.

R- The Quickness

]This is also a really good engage tool and if paired with his W and E, it can cripple and kill enemies. If you use W to go in and then activate his R, you can charm the enemie sthat you engaged with and then use E to safely disengage.