After choosing your starter Pokemon, go home. Your will tell you to go to Sandgem town to talk to Professor Rowan. There is plenty of tall grass on the way so fight any wild Pokemon that you encounter to level up your Pokemon to at least around level 7. Around the second patch of grass, you'll see a person standing in the top near the corner that will give you a free Potion if you talk to them. After receiving your Pokedex and touring the town, go to the store and buy as many Pokeballs as desired. Lucas or Dawn also give you some after one of them shows you how to catch wild Pokemon.