well i made this i hope its good beacuse im new to this.

Sona is great support beacuse of her auras.in the skill i would max Q fast beacuse ad carry needs damage so he/she can last hit minions more quickly and does more damage to apponents.
i give heal second beacuse ad neeeds dmg more than heal at the beginning.And E the speed buff
ad carry doesnt need 500 speed (well maybe when low hp),but not all the time.I max E last.

in masteries i give it 0/9/21.I give it 21 beacuse u need mana regen if u play sona you know u need mana a lot.Otherwise u have to go back every 5 min.And u need exp beacuse ur support and u have to go back more often than ad carry(exept if ad carry lacks of mana).

in runes i take armor,magic resist and speed.Armor and magic resist are needed beacuse if your opponents cant kill the ad carry and u are more slow they will kill you so u need speed and armor/magic resist.

for summoner spells i take flash(no need to talk why i think)and clairvoyance to see where they are and if they went baron/dragon.u can take exhaust instead clairvoyance.

for items take fairy charm and 3 wards and 2 health and 1 mana potions fairy charm for mana regen as is mana potion too.2 health potions are for not wasting mana to heal yourself.
then i take philosofer stone but buy wards always when u go back.i buy boots after the philosofer stone and next back i buy hearth of gold.Philososfer stone and hearth of gold are for more gold.then i buy shurelias reverie beacuse in teamfights u need speed.U can use it to flee or to attack/rush.aegis of legion great item for your alleys and for your W more armor and magic resist.I buy soul shroud for mana if u have ad carry that needs mana.
i buy randiums omen or locket of iron solari.U chose randiums omen slowes attack speed and its perfect against teemo and kog maw and corki.locket of iron solari gives u sheild.if u have spare gold you can buy both.