Instructions
Play through the game until you have at least 10 types of Pokemon. Types include attributes such as Electric, Bug, Flying and Water. Just because you have 10 Pokemon does not mean you have 10 different types. Some Pokemon share a type, such as Abra and Drowzee, which are both Psychic types.
Head south on Route 2, which can be found on the other side of Diglett's cave. Walk past the house and continue on until you find trees that you can cut. Cut the trees and continue heading south.
Enter the large building past the clearing. Inside, you'll find one of Professor Oak's aides.
Speak with the aide. As long as you've captured at least 10 types of Pokemon, he'll give you the Flash ability.