After you told your Mum about your adventure, head to Route 202 and Dawn/Lucas will teach you how to catch a Pokémon. After that, he/she will give you 5 Poké Balls for free. You can now catch your favourite Pokémon and start collecting it to make a strong team!
Some Pokémon can be found in different time of the day. For example, on Route 202, Kricketot can only appeared in the morning and night. It does not appear in during the day. So if you want to catch the Kricketot, make sure you play during that time. You can also change the time on your Nintendo DS to catch it.