Use the format listed in the game
Little Cup formats generally cap each Pokémon at 500 CP. The classic rules require a Pokémon that can evolve and has yet to evolve. Variants may use their own rules, so open the exact cup details.
Save a low-CP copy
A power-up can move a Pokémon over the cap. Check the final CP before confirming, and build with a backup that covers a weakness in your starter.