The Cable Club is the place to go when you want to link up with another GameBoy. Cable Clubs are found in the Pokemon Centers. You'll want to do this for two reasons, to fight and to trade. When you fight, you can take on your friend and build up experience points. Once your Pokemon is up to a certain level, sometimes it will evolve into a more powerful creature.
Another reason is to trade, and if you want to collect all 150 Pokemon, you'll need to do some serious dealing. Most Pokemon will evolve faster when traded (though they tend to be weaker), and some Pokemon will evolve only if you trade it to another GamePak. There are also two versions of the game, a Red Version and a Blue Version. While both games are identical, one Pokemon that is very common in one version may be rare in the other. The versions will even have monsters exclusive to that version, meaning a monster you find in the Red Version may not be found in the Blue. If your friend has the other version, you can work together to collect all the Pokemon! Some hardcore gamers may even consider buying both versions!