Sona is a VERY beginner friendly support champ, she's very flexible and works great with many different adc's, now just because she's flexible doesn't mean she's foolproof, try out combinations with friends and see what works best,

Getting started

As I've stated before, Sona is extremely beginner friendly for those new to the role of support, she has great peel, great utility and some decent CC in her ult, Sona is generally meant to keep the team alive, not as much in the way of killing in 1v1's unless against the opposing support or cleaning up, that is until late game where she can easily shred champs like Malzahar if you play your cards right, but aside from that, Sona isn't just a pretty face, she can also carry fairly well, make sure you level up "Aria" (W) first as it is her main supporting ability.

combinations

Sona is a champ who has a passive called "Power chord", upon activating 3 spells her next auto attack will deal an additional effect,

here's a quick guide on combo's with Sona,

If a combo ends with Hymn of Valor (Q):
your next auto attack will deal additional 40% magic damage.

If a combo ends with Aria of perseverance (W):
your next auto attack will suppress damage the effected target will deal for a short time (suppressed by 20%).

If a combo ends with Song of celerity (E):
your next auto attack will slow an enemy by 40%

Like any champ, Sona combo's well with certain champs, here are the few I know of:

Tristana,
Draven,
Ezreal,
Caitlyn,
Vayne,
Graves,
Miss Fortune.