This is for newbies only! When you pick your first starter and went on to challenge the pokemon diamond world you must first capture at least 3-4 differrent pokemon so you can have many backups when your battles become intensified and your starter has fainted. Before every gym battle you must first level up your pokemon up to the point that you exceeded the levels of the gym leader's pokemon so you can beat them easily and don't have the trouble of repeating the same battle always. This rule also applies for your rivals. Hope this helps you newbies out.