Instructions
Go to Saffron City in the Kanto Region after defeating Red at Mount Silver.
Find the Silph Co. building and head inside.
Speak to Steven Stone on the main floor of the Silph Co. building.
Select a color when prompted by Steven Stone. Which color you select will determine which Pokemon you receive -- red will yield Torchic, blue Mudkip and green Treecko.
Nickname the Pokemon if you desire. You now have one of the "Ruby," "Sapphire" or "Emerald" starters as one of your Pokemon.