Where is Viridian Forest in Pokemon Yellow?

Answer 1

Start your game and walk into the grass atop the town you start in. Then you start to follow Prof. Oak into his Lab. Pick a Pokemon after he is finished talking and battle your rival, and so on.

You may win or lose for this battle; it doesn't matter. Then, go through the grass atop the first town again (this time you will be able to go through it). When you arrive at the next town, you must walk into the Poke Mart. The clerk will tell you he has a Parcel for Prof. Oak. Then after you receive it you will have to go back to Pallet Town. Go into Prof. Oaks' Lab. Then you give him the Parcel (this is where your rival walks in and you both get your first Pokedex). Go to the house above the Lab. Your rival's sister will give you a Town Map. Then you go up to the second town again and go all the way up to the left a bit. Then you will find Viridian Forest.

Viridian Forest is North of Viridian City and South of Pewter City. You can only access the forest from the Viridian City entrance (found on Route 2) on your firt visit, but you may revisit it from the Pewter City entrance (on Route 2 as well).

Diglett's Cave (accessed on Route 11 - just outside of Vermillion City) also leads to the Viridian Forest, although you will need cut to access the end of the path and the entrance is just North of where you end up (or South if you chose to use Cut on the Pewter City side tree).