Some Pokémon don't Evolve till you give them a Stone. They are:

FS = Fire Stone
TS = Thunder Stone
WS = Water Stone
LS = Leaf Stone
MS = Moon Stone
Hint: You can get the stones in the Mart in Celadon City.

Before = After
Pikachu + TS = Raichu
Nidorina + MS = Nidoqueen
Nidorino + MS = Nidoking
Clefairy + MS = Clefable
Vulpix + FS = Ninetales
Jigglypuff + MS = Wigglytuff
Gloom + LS = Vileplume
Growlithe + FS = Arcanine
Poliwhirl + WS = Poliwrath
Weepinbell + LS = Victreebel
Shellder + WS = Cloyster
Exeggcute + LF = Exegguter
Staryu + WS = Starmie
Eevee + WS = Vapereon
Eevee + TS = Jolteon
Eevee + FS = Flareon