Either go back to your house and use wardrobe in your room or go into any pokemon center. There's "changing booth" on the ground floor in every single one of them. There are couple more places to get dressed, but I guess if you just started, those are irrelevant for now.
Edit:
But for completion sake... there's cabin where you can dress/undress in North Kindling Woods and every time you return towel in any of the current 3 hot spring areas, you get dressed too. Although the hot springs worked like that before clothing system got updated, so I'm not sure it still works this way.
Edit:
But for completion sake... there's cabin where you can dress/undress in North Kindling Woods and every time you return towel in any of the current 3 hot spring areas, you get dressed too. Although the hot springs worked like that before clothing system got updated, so I'm not sure it still works this way.
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