Route 2

    Early in the game you can only access about half of route 2.  This forces you to pass through Viridian forest in order to get to Pewter city.  After you learn the cut ability you can access the right side area and Diglett's Cave.  There are Rattatas and Pidgeys in this area in both games however there are Weedle's only in the red game and Caterpies only in blue.

Viridian Forest

    Viridian forest is the first place where you can fight trainer battles with someone othe than Gary.  Ther are several Bug Collector trainers in the forest.  You shouldn't hav emuch problem against them, the hardest fight would probably against Kakunas since they can harden for protection and poison your pokemon.   You can find Caterpie, Metapod, Weedle, Kakuna, and Pikachu in Viridian forest.  

After you make it through the maze, you return to route 2 on the other end and can enter Pewter City.