How to Beat the Gym Leaders in Pokémon Fire Red

Train your Pokémon up. You should have a starting team that is like this: Starter, Pikachu, Spearow/Pidgey, Mankey, Beedrill/Butterfree, and another Pokémon.

To beat Brock, follow this step. Your Pokémon should be level 9 or more. This is a good chance to level up a weakling or two. Use Metal Claw if you have Charmander, Bubble or Water Gun if you have Squirtle, or Vine Whip if you have Bulbasaur.

To defeat Misty, use a Grass or Electric-type. Also, your Pokémon should be around 20 to 25 (Bulbasaur or Pikachu).

Use a Diglett against Lt. Surge. You can find a Diglett in the nearby Diglett's Cave. You can even use Dugtrio! Also, your Pokémon should be in the late 20's.

To get Erika beaten, use this strategy. If you picked Charmander, than you've got nothing to worry about! If you chose Squirtle or Bulbasaur you can use a Pidgey or Pidgeotto, or use a Spearow or Fearow. Otherwise use a Fire-type (Vulpix, Flareon, Growlithe, Arcanine, Ninetales).

To beat Koga, use a Psychic or Ground-type. Either that or a very powerful Pokémon. If you're using a Ground-type, be aware that Koffing and Weezing have the Levitate ability, preventing you from using Ground-type moves against them.

To beat Sabrina up, use a Ghost-type (Haunter or Gengar). Or you could evolve Weedle up into Beedrill, who knows Pin Missile (Jolteon also learns this move). If this fails, use a Pokémon with a strong physical move. Kadabra and Alakazam may have a high special attack and speed stat, but they don't fare well against physical attacks due to their low defense stat. Pokémon such as Fearow and Snorlax are good physical attackers.

To get Blaine down, bring Diglett (which should have evolved into Dugtrio by now) back, or use a Rock, Ground, or Water-type.

Finally, to defeat Giovanni, use a Water or Grass-type. You could also bring a Psychic-type to deal with Nidoking and Nidoqueen.