Instructions
Turn on your Nintendo DS with "Pokemon Pearl" inserted. Load your current game.
Press "L" to bring up your pause menu. "Start" doesn't pull this menu up, as it did in many previous Pokemon games.
Navigate to "Pokemon" and press "A." With your party's first Pokemon highlighted by default, press "A" twice to pull up that Pokemon's summary.
Remember what the summary says under "Type." For example, the starter Pokemon Piplup is a Water-type Pokemon. Press "B" to return to the Pokemon selection menu. Navigate to another Pokemon in your party and press "A" twice to bring up its summary. If the type is something other than Water, this Pokemon is of a different type than your first. Press "B" to return to the Pokemon selection screen again.
Press "A" over the Pokemon you'd like to switch with your default Pokemon, the first Pokemon you fight with when you enter a battle. Press "Switch" and select the first Pokemon, the one in the upper-left corner. It should now be switched and, if you paid attention to their types, the first Pokemon will be of a different type than the previous first Pokemon.