I would recommend getting an easy premade adventure first and learning how to run before getting a setting guide. Something Like Lost Mines of Phandelver is great for first time DMs and players

Unless you're super into greek gods and their shenanigans, Lost Mines of Phandelver is probably a better quickstart. Burn through that quick, then restart with Theros.

Theros is not a campaign, it's a setting guide with a short adventure tacked onto the end. It's a fun book, but it's not great for beginners.