Good book or series?

Okay where do I START on Discworld. I love Good Omens and my best friend raves about Discworld so I think I'd like it. But there's just too dang many books.

We know you are here, our brothers and sisters . . .
Pressia barely remembers the Detonations or much about life during the Before. In her sleeping cabinet behind the rubble of an old barbershop where she lives with her grandfather, she thinks about what is lost-how the world went from amusement parks, movie theaters, birthday parties, fathers and mothers . . . to ash and dust, scars, permanent burns, and fused, damaged bodies. And now, at an age when everyone is required to turn themselves over to the militia to either be trained as a soldier or, if they are too damaged and weak, to be used as live targets, Pressia can no longer pretend to be small. Pressia is on the run.

Burn a Pure and Breathe the Ash . . .
There are those who escaped the apocalypse unmarked. Pures. They are tucked safely inside the Dome that protects their healthy, superior bodies. Yet Partridge, whose father is one of the most influential men in the Dome, feels isolated and lonely. Different. He thinks about loss-maybe just because his family is broken; his father is emotionally distant; his brother killed himself; and his mother never made it inside their shelter. Or maybe it's his claustrophobia: his feeling that this Dome has become a swaddling of intensely rigid order. So when a slipped phrase suggests his mother might still be alive, Partridge risks his life to leave the Dome to find her.

When Pressia meets Partridge, their worlds shatter all over again.