Here are some tips on training Pokémon for different goals you may have. If you
want to be a great trainer so you can beat all of your (human) challengers, follow the
top list of tips "...Always Win in Two Player Battles:". If you are just trying to collect
all 150 Pokémon and get through the game, follow the bottom set of suggestions. I am
personally trying to become an excellent trainer to win in battles (like Ash).

To Become an Excellent Pokémon Trainer
and Always Win in Two-Player Battles:

• Choose Squirtle as your starter Pokémon. He is the strongest of all three
and will grow-up to be one of the 10 most powerful Pokémon.

• YELLOW:
Save all of your Rare Candies until your Pokémon reach high levels (50+ or 60+). Each time
your Pokémon gains a level, it will need more experience points than before to reach the next level.
When your Pokémon has a high level, the Rare Candy will be more valuable.
RED & BLUE ONLY (R&B):
Save one Rare Candy until the end of the game, then use the 99 Item Code on the item. Use
all the resulting Rare Candies on your Pokémon.

• Save all of your TMs until your Pokémon learn all the moves they can on their
own. By the time they learn moves by themselves, you can decide which attacks
will be the most suitable for your different conditions.

• The highest level a Pokémon can reach is level 100. After that, it
will cease to grow, even if it gains more experience in battle!

• When you receive all eight of your badges, constantly go to the Pokémon League
to train your Pokémon and get more money. NOTE: Do not buy any more supplies!
 Your Pokémon should be strong enough that they won't need them and you will get
more money faster.

• Take at least three Pokémon above level 50 to the Pokémon League.

• Once you have lots of money ($200,000 - $999,999), go to the department
store at Celadon City and buy a lot of vitamins and minerals (HP up, Protein, Iron, etc.)
for your Pokémon.

• Keep a variety of Pokémon in your main list.

• If you teach your Pokémon a variety of moves, you won't have
any major disadvantages. For example, I trained my Pikachu to
defeat an Onix by teaching Pikachu Flash and Swift. That way,
I keep the Onix from hitting my Pikachu with Flash, while Pikachu 
continues to attack him with Swift until he goes down.

• THIS IS A VERY IMPORTANT STEP. 
If you don't have tons of Rare Candies, train your Pokémon to level 100 one by one.

To Be a Good Trainer,  Get Through the
Game, and Collect all 150 Pokémon as fast as
Possible:
 

• Choose Squirtle as your starter Pokémon. He will be the best to defeat the
beginner Gyms, and will be very powerful later on in the game.

• As soon as you get a Rare Candy, use it on a Pokémon you know will
be a permanent member of your main six Pokémon, like the starter Pokémon
you choose.

• If a TM seems like it could be useful in your current conditions, teach it
to a Pokémon immediately.

• Keep a variety of Pokémon to battle the different types you will encounter in the game.

• Take as much time as you need searching for Pokémon in all areas of the game.

• Always keep a full stock of PokéBalls.