Instructions
Bulbasaur as a Starter
Turn on the Game Boy system with the "Pokemon: Red Version" cartridge inserted in it. Select "New Game" from the game's main menu.
Read the game's opening monologue and exit your character's house. Attempt to run north, out of town. You will be stopped by Professor Oak and taken to his house.
Talk to Professor Oak until he offers to give you one of three starter Pokemon.
Move your character in front of the Pokeball that contains Bulbasaur and press the "A" button. When asked if you are sure this is the Pokemon you would like, select the "Yes" option.
Bulbasaur Through Trade
Plug your Game Boy system into another Game Boy system with a Pokemon game cartridge via the Game Boy link cable. Make sure the person with whom you are trading has Bulbasaur as one of his Pokemon.
Go to the "Pokemon Center" in the game and then on to the "Trade Center."
Select the Pokemon you wish to trade on your Game Boy and select Bulbasaur from the other Game Boy. Confirm the trade.
Allow several seconds for the trade to finish.