Pokémon Red and Blue/Viridian Forest
The Viridian Forest is smack-dab in the middle of Route #2, the route that runs between Viridian City and Pewter City. Pokémon are plentiful within its borders, and many trainers come here to capture new specimens for their collections and to challenge other trainers. It's easy to lose yourself in Viridian Forest if you don't know where you're going, so be careful!
 Through the Woods
Viridian Forest serves as your first "dungeon" stage. It's hard to get lost here, but first timers may wish to explore it slowly to get a feel for the layout. As you cross Viridian Forest, you'll be attacked by both wild Pokémon and other trainers. While encounters with wild Pokémon are random, trainers will only come at you when you cross their line of vision, so you have a degree of control over when they challenge you. Unfortunately, they are usually placed in positions that give you no choice but to pass in front of them, although a few can be avoided by walking around them.
If you are having trouble defeating trainers in a given area, it probably means that your Pokémon aren't strong enough and need to be leveled up more. The highest level Pokémon that a trainer in Viridian Forest has is level 9 (level 10 in Yellow), so as long as most of your Pokémon are level 8 or above, you should be in good shape. If you caught Nidoran and Spearow on Route 22, you're golden - Nidoran's Poison-type means you can soak up Poison Stings from Weedle, and Spearow's Peck has a type advantage against Bug-types.
The Pokémon that you encounter in the forest may seem tough, but most of them won't be. Wild and trained Kakuna and Metapod can't attack, only using Harden to raise their Defense (which can make for some long battles that wear down your Pokémon's PP, so be careful). These two also give lots of experience when defeated, so use your weakest Pokémon when fighting them. Never try to capture a wild Metapod or Kakuna. Instead, capture Weedle and Caterpie and level them up so that they evolve into Kakunas and Metapods while retaining their old moves. They evolve fast: both will be at their third stage of evolution by Level 10! Definitely capture a Caterpie and raise it to Butterfree, especially if you chose Charmander as your starter.
Pikachu are extremely rare in Red and Blue (and non-existent in Yellow, since you already have one) and will take extensive periods of walking around through grass to find them. It isn't necessary to capture one, but they can deal with Flying-types and Water-types with ease, and it'll be awhile before you can catch another Electric-type Pokémon.
As you enter the forest, take the path on the left side of the large tree ahead to find a hidden Antidote, and directly in front of Trainer #5 is a hidden Potion.
|#10 Caterpie||#13 Weedle|
|Butterfree that can gain status causing moves such as Stun Spore and Sleep Powder, as well as a basic Psychic attack in Confusion (this is a damage attack). Butterfree can be extremely helpful in the early portion of the game. However, Butterfree is quickly out-classed by other Pokémon such as Bellsprout and Oddish as a status-inflicter, and Kadabra and Hypno in terms of Psychic power. It gets its moves earlier in Yellow, and even learns a new one, but it's still not much of a late-game Pokémon.||Beedrill. Beedrill learns the best Bug-type attack, Twineedle, at level 20, but Beedrill's Poison-type and low stats make it pointless as an anti-Psychic Pokémon. Good until around Rock Tunnel, where you may want to consider finding a replacement.|
|#25 Pikachu||#17 Pidgeotto|
|Vermilion City. It gets Swift, but at the expense of Double Team, Slam, and even Light Screen later on. The big advantage is that Pikachu can evolve, giving it good stats and making it much stronger in the long run. Definitely a good Pokémon to pick up, particularly if you chose Bulbasaur and need a Flying-type counter.||Pidgey, it will evolve on its own at Level 18. But, if you have Yellow, you don't need to wait! Rare, evolved Pidgeotto can be found in Viridian Forest. These are superior to Pidgey because they have higher stats right off the bat, making them harder to catch but easier to raise and battle with.|
|#11 Metapod & #14 Kakuna|