When you get to a Pokemart, you will buy items eventually. So this is how you buy items.
1) Press new topic and says "Your name Buying Items"
2) Then you say what you bought, so lets say you have $300. You want to buy a Pokeball, you just say "300-200 = 100, buying 1 Pokeball."
3) Buy till you drop I guess, have fun shopping.