In the top right of your interface you'll see an arrow you can extend, after extending that panel you can click on the grass icon which will show you any Pokémon that are available in your current area (so long as you have seen them).
If you want to see where most of the Pokémon are without using this feature or for areas you are not currently in, the links below are useful:
Kanto Guide
Johto Guide<i></i> :Cool: