You have access to everything right off the bat. Talk to nurse Joy to get everything (pokeballs, rare candies, held items, repel) if you haven't yet. Change your pokemon's abilities, nature, EVs (unless you turn off EVs in the settings), and movesets in the pokemon's summary menus. If you're not sure how, just look around the screen until you see the instructions on what button to press. It's really just about building your team for each challenge.
You have access to everything right off the bat. Talk to nurse Joy to get everything (pokeballs, rare candies, held items, repel) if you haven't yet. Change your pokemon's abilities, nature, EVs (unless you turn off EVs in the settings), and movesets in the pokemon's summary menus. If you're not sure how, just look around the screen until you see the instructions on what button to press. It's really just about building your team for each challenge.