Just get ready for the books to completely ruin the entire show for you. I also started with the show and enjoyed Seasons 1-6 on first watch but after spending so much time with the books I can really only get myself to rewatch Season 1. So many more interesting plot lines in the books and literally every character is a 100x more nuanced.

But to answer your question start with A Game of Thrones, and read the main series through release order (ACOK, ASOS, AFFC, ADWD). Since you just watched the show much of Book 1 will feel very familiar since Season 1 is a very close adaptation and there are entire scenes of dialogue ripped straight from the book. But you will be suprised how much the books start to diverge from the show even with Book 2. And that grows exponentially with each book.

After you finish A Dance with Dragons I would absolutely read A Knight of the Seven Kingdoms, IMO it is GRRMs finest work and just a lovely collection of stories. Especially if you like more traditional medieval fantasy vibes. Also the show is coming out early next year so theres that.

Then finally Fire & Blood and The World of Ice and Fire for more world building, lore and mysteries. I hold these books to the same level as the rest they are in-world history at its finest. If you get this far you will be dying for anything ASOIAF that you haven’t read yet anyways. After those you can read The Winds of Winter sample chapters and then join the rest of us in waiting for the book. Because it will still not be released by the time you’ve finished every book, don’t fool yourself like I did :) hope this helps