The First Law

Start with the First Law trilogy (Books 1–3), then the three standalones in any order, then the Age of Madness trilogy. The standalones and Age of Madness reward readers who know the original trilogy.

Yes — Abercrombie is considered the defining author of grimdark fantasy. His books feature morally grey characters, graphic violence, and a deeply cynical view of heroism and power. If you appreciated the darker elements of Game of Thrones, you will likely enjoy Abercrombie.

Technically yes, but not recommended. The Age of Madness is set 30 years after the original trilogy in the same world, and many characters, consequences, and references from the trilogy appear. Reading the trilogy first dramatically increases the impact.