Pokémon Red and Blue/Route 2 & 22

'These routes are a good place to collect more Pokémon. Route #2 (shown below) is full of Pidgey and Rattata, along with a specimen or two that are rarely found in this area. Remember to weaken a Pokémon in battle before using a Poké Ball on it, or else it will be able to break free. The first time you come through Route #2, head into Viridian Forest to continue your journey.'

Gary Awaits
It's a good idea to take some time and train your Pokémon in Route #22. If you're playing the Yellow version, you'll have the opportunity to catch a Mankey. Be sure that you do. Otherwise, be sure to pick up whichever Nidoran is more prevalent in the version you're playing, both if you're trying to complete your Pokédex. If you also stumble upon a Spearow, it makes a nice addition to your collection. If you would like to earn extra EXP (and 280), you can challenge your rival in Route #22 by heading west towards the pond along the top path. Before you do, make sure that you have a few strong Pokémon, and that they're fighting fit! You may wish to until after you've traveled through Viridian Forest to conduct this battle... or skip it enitrely.

When you're done in Route #22, make your way back to Viridian City and exit through the north path to reach Route #2. Much of this route is inaccessible to you early on in the game, but that will change in time. For now, you're only concern is reaching the entrance to Viridian Forest. Don't bother hunting for Caterpies or Weedles out here, they're more plentiful in the forest.