You'll need to be in the Kanto Region.
Once you reached Pewter City via Diglett's Cave, just head into Route 3. You'll see 4 rocks, it won't be too hard to miss. You can change forms there.
To change to Normal Forme, use the top-left Meteorite. To change to Attack Forme, use the top-right meteorite. Defense Forme is obtained by using the bottom-left meteorite and finally, Speed Forme is found by using the bottom-right meteorite