Recently, Pokémon GO introduced a competitive system where players from around the world can compete against each other and win rewards. The GO Battle League is limited to 5 fights per day and allows players to collect Star Dust, Rare Candy, and meet rare Pokémon.
And it's now official: Pikachu Libre will be one of the rewards of the GO Battle League. Please note, however, that only participants in the first Competitive Season will be able to access the reward.
For the moment, the Pre-Season is underway. We will have to wait until mid-March (probably) for this pre-season to end and make way for Season 1.
To obtain the Pikachu Libre, all you have to do is participate in Season 1.
Concerning clothes and accessories for your avatar, you will have to win some fights to unlock them.