You can find Korok seeds before meeting Hestu, but if you want to start trading them for inventory slots, you’ll need to locate him first.
Locations n. 1 – Dueling Peaks Tower Region
Head north from Dueling Peaks Stable towards Kakario Village, following the main story, and shortly after crossing the Kakariko Bridge, you will see Hestu standing next to a tree on the right side of the path.
It will ask you to retrieve its maracas, and after completing its mission, you can start upgrading. The first level upgrade for each category costs one seed, the second level costs two, and so on.
After unlocking some additional slots for you, Hestu announces his departure to the Korok forest, so you’ll have to track him down again for more inventory space.
Location n. 2: any region
Hestu’s next location will be the first stable you visit after you move from the road to Kakario Village.
In my case, this was the Riverside Stable, right across from the Wahgo Katta Shrine. For others, it appeared in Wetlands Stable and Woodland Stable. Wherever your next stable is, it will be close by.
Location # 3 – Korok Forest, Woodland Tower Region
The forest is in the Woodland Tower region, and this is Hestu’s third and final location.
To get to the forest, you will have to enter the Lost Woods to the southeast. At the entrance there are some ruins next to a torch. Follow the torches until it looks like your path is over.
Head west and you will come to a pair of torches next to a tree that has a chest hidden inside. Grab the loot and then grab the moving torch on the ground.
Follow the embers of the torch and it will lead you to the entrance of the forest.
Hestu is to the right of the Great Deku Tree.
If you collect all 900 seeds, you will receive Hestu’s gift, allowing you to activate Hestu’s graceful dance routine anytime you wish.
It seems like a fair reward for all that work.