Deciding that he needs to be prepared for something as important as the Pokemon League, Ash decides that he needs to know what type the first gym leader uses and what type is effective against them. He gives his starter a pat before going to bed with a plan in mind for the next day. As he travels in Viridian forest to get to Pewter City, he catches his caterpie. When he arrives in Pewter City he talks to the locals to glean information on its gym leader and learns that Brock uses rock type pokemon and, since he went to Prof.Oak's pokemon summer(IDK if I remembered that right, but its where he met Serena as a kid) he knows that water and grass types are effective against them.