You can access the ranked ladder for free after earning "10 First Wins of the Day" in Unranked or Versus AI matches.
The full first chapter of the single-player story is entirely free.
Below is an overview of what is currently free and where you can safely get the game. What is Free in StarCraft 2?
Since 2017, Blizzard Entertainment has made a significant portion of the game accessible without any purchase or product key:
All community-made custom games and maps are free to play. What Still Requires Purchase?