The Gym here starts out with a JR. Trainer with a Level 11 Diglett and Level 11 Sandshrew. You probably never met these two, but a good hint is that Diglett will be strong against your Pikachu, so use water on it. Water also works well on the Sandshrew. Plant does too. Brock, the Gym Leader has a Lv .12 Geodude and a Level 14 Onix, 2 Rock-type Pokémon. Water works very well on these. Be careful for the Onix's BIND (a multiple-hit move in which your Pokémon is immobile during it) and BIDE, a move where it absorbs damage for some turns and returns it double. When
it does BIDE, I suggest either beating it very quickly or do non-damaging attacks like TAIL WHIP or GROWL, so you are debilitating him without taking damage. If you defeat him, Brock will congratulate you and give you BIDE, TM 34. I suggest giving it to a Pokémon with high hit points, and it is best used against Pokémon with high defense and high offense, because the damage done doesn't care about the enemy's defense at all. You can look around the museum, but the $50 it costs is unreasonable, since you can't get anything there except looking at fossils of Aerodactyl and Kabutops, two extinct Pokémon which you might get late.