The secret battle star for week 1 in Fortnite Season 9 is located East of Mega Mall. The exact location is where the coordinates i5, i6, j6 and j6 meet. See this map for the exact location:
You can collect the secret battle star for week 1 at the above location. You can only see the secret battle star if you complete all week 1 challenges and unlock the week 1 loading screen. The secret star is only visible if you have a battle pass!
Secret star week 1 location video
This video will show you the location of the star.