Regardless of which Starter Pokemon you pick in Pokemon Sword and Shield you will have the opportunity to grow this Pokemon into a stronger version of itself by leveling it up. The three starter Pokemon are a Grass-type, Fire-type, and Water-type each with unique abilities.
No matter if you choose Grookey, the Grass-type Monkey, Scorbunny the Fire-type Rabbit, or Sobble the Water-type Lizard, the Pokemon will evolve at the same time and do so automatically. The Pokemon will evolve at level 16 and then will evolve again at level 36.
Grookey -> Thwackey – Level 16
Thwackey -> Rillaboom – Level 36
Scorbunny -> Raboot – Level 16
Raboot -> Cinderace – Level 36
Sobble -> Drizzle – Level 16
Drizzle -> Inteleon – Level 36
There is nothing needed on your part to perform the evolution of your starter Pokemon. Simply continue to level up these Pokemon by having them in your party and you will reach their final form in no time.