This depends on if you are asking what the first required story boss is within the game or merely the first boss that you encounter in general. If you are talking about the first boss you encounter before even traveling to The Lands Between; this would be the Grafted Scion. However, this is a very tough fight, and by design, the developer makes this boss near impossible to defeat as players are meant to die to progress. After this, in The Lands Between, the first boss the player will encounter is the Soldier of Godrick. This is a very underwhelming boss, again by design, as this is to serve as a tutorial for players to witness how boss battles play out. Then lastly, if you mean required boss battles to progress the story, it is likely that the first boss of this nature you will fight is Margit, the Fell Omen. They are a very tough boss, and they will test even veteran players to their limit right from the word go.