When you get to Viridan Forest, go to the left and just wander around in the grass. After a couple of battles, you will finally battle a wild Pikachu! Pikachu is found at level 3 or 5, most commonly 3. This pokemon is more common in the red version than the blue version. Lower it's HP to around 4, and throw a pokeball at it. This electric pokemon is extremely useful and powerful and starts with the attacks Thundershock and Growl. This pokemon has an extremely high speed but low defense.
The other pokemon in the forest are all bug and/or poison typed and are: Weedle, Kakuna, Caterpie, and Metapod. Only catch a Weedle and Caterpie because Kakuna and Metapod are their evolved forms and are particulary useless because all they have are Harden, which raises their defense. You can also save more pokeball money by doing so. Both Weedle and Caterpie evolve into their evolved form at level 7 and finally to their final form, Beedril and Butterfree who are both very powerful at level 10.
Gain a couple of levels for your pokemon, especially pikachu and a pokemon that is strong against Gary's starting pokemon. Make sure they're both around level 10. Go back to viridian city and go left to the area where you caught nidorans. Go above that area and start going left. Gary should approach you and challenge you. Before you fight him, press start to go to your menu and go to Pokemon. Put pikachu as your first pokemon in line-up so pikachu will fight your enemies. Gary starts with a level 9 pidgey and a level 8 "......." (starting pokemon). Use Pikachu's Thundershock and Pidgey should be dead in 2 shots. If Gary uses Bulbasaur, he has Tackle, Growl, and Leech Seed. Use any pokemon against him except Squirtle and Pikachu. If he uses Charmander, he should have Scratch, Growl, and Ember. Use any pokemon except Bulbasaur. If he uses Squirtle, definetly use Pikachu. By the way, make sure you battle him before you battle the first Gym leader in Pewter City, Brock. If you beat Brock before Gary, you cannot fight Gary. Go back to Viridan Forest and follow the path to Pewter City. If you haven't already battled the numerous bug catchers, do so. The bug catchers usually have around 2 Pokemon, which are all bug such as weedle, kakuna, caterpie, or metapod. If you have a Charmander, you can use Ember to wipe them all out. There are some items on the ground that look like Pokeballs. Pick them up.
