The first Groot Awakening challenge is your first step to earning the Rocket "Battle Brothers" emote. Fortnite season 4 is offering up a bunch of Marvel characters as skins in the battle pass (like Wolverine and She-Hulk), but to get their special emotes and superhero skins, you'll need to complete each character's Awakening Challenge.
For Groot's first challenge, you need to find where to plant a seed on a heart-shaped island. Here's exactly how to do that.
Heart-shaped island location
The heart-shaped island you need to visit is located on the west side of the Fortnite map, as highlighted in the map above with my camera icon.
Here's what the heart-shaped island actually looks like. It's not all that big, and it's easy to miss, but you'll find it among the other small islands/rocks on the coast west/southwest of Sweaty Sands.
Once you're on the heart-shaped island, you should see an interact prompt at the center of that dirt circle. Interacting with it plants a seed, obviously.
As far as challenges go, this is a pretty simple one. Next up on the Groot Awakening Challenge list is to emote at a friendship monument.
Epic also recently added the Panther's Prowl location, based on the incredibly popular Black Panther, and fans have turned it into a memorial to pay tribute to the late actor Chadwick Boseman.