The evolution system on Pokémon GO is a completely new concept. No PC boxes filled here but candy offered for each capture! Thus, to evolve a Pokémon you need a certain number of candies distributed by this same Pokémon when it is captured.
To develop its Rattata in Ratatouille You must therefore capture around ten Rattata and then give the Rattata candies obtained.
Candy can also be used to “recharge” your Pokémon, i.e. increase its CP. If you are short on candy, you can get some from Professor Willow by “transferring” him the Pokémon you have in excess.
There are also Pokémon that can only evolve through items. These were added with the advent of 2G.
Finally, since the arrival of 5G, it is possible to evolve certain Pokémon by trading, as in the original games.
During its evolution, the PC of your Pokemon will be modified, as well as its attackswhich will be drawn at random again.
You can find all the information about the battle system, Pokémon and attacks of Pokémon GO in the dedicated guides.