You can find Korok seeds before encountering Hestu, but if you want to start trading them in for inventory slots, you'll need to locate him first.
Locations #1 - Duelling Peaks Tower region
Head north from Dueling Peaks Stable towards Kakario Village, following the main story, and shortly after crossing Kakariko Bridge, you'll spot Hestu standing next to a tree on the right side of the path.
He'll ask you to retrieve his maracas and after completing his quest, you can start upgrading. The first level upgrade for each category costs one seed, the second level costs two, and so on.
After unlocking a few extra slots for you, Hestu announces his departure for Korok Forest, so you'll need to track him down again for more inventory space.
Location #2 - Any region
Hestu's next location will be the first stable you visit after he moves on from the road to Kakario Village.
In my case, this was the Riverside Stable, directly opposite the Wahgo Katta Shrine. For others, he's popped up at Wetlands Stable and Woodland Stable. Wherever your next stable is, he'll 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 reach the forest, you'll have to enter via the Lost Woods to the southeast. At the entrance, there's are some ruins next to a torch. Follow the torches until it seems like their path has ended.
Head west and you'll come to a pair of torches next to a tree that has a chest hidden inside. Grab the loot, and then grab the mobile torch on the ground.
Follow the embers from the torch and it will lead you to the entrance of the forest.
Hestu is off to the right of the Great Deku Tree.
Should you collect all 900 seeds, you'll receive Hestu's Gift, which lets you trigger Hestu's snazzy dance routine any time you want.
Seems like a fair reward for all that work.