some People begins with the Hymn of Valor cause its good for your ADC with the damage boost, but I think its important to heal fast in first fights. Many people skills the Song of Celerity too late I think, but its very good to have the movement speed qickly and for slow.
Spells and Choices
your spells like Exhaust is very good for your ADC because of the slow, but you can take the Heal too.
Start: Your can start with Faery Charm, 4 Sight Wards and one Heal Potion. Or you can start with Mana+ Heal Positions and Vision Wards+Sight Ward.
First Port: If u have not enough Gold, u can buy Philosopher´s Stone and Sight Ward´s.
If u have an annoying Enemy, buy fast the Oracle´s Elixier, cause most of them buy the vision ward too, and destroy urs....so save ur Gold and buy the Elixier.