You can sign into Pokemon Go using your Google Account or a Pokemon Club account. Both are tracked individually.
Choosing a Starter
After that, Professor Willow will tell you about Pokemon being nearby. This will give you the chance to catch 1 of 3 starter Pokemon. While the CP is unknown, you will easily be able to catch it.
When you initially start the game, you are given the choice of Bulbasaur, Charmander and Squirtle. If you ignore them and run away from them 4 times, you will see the 3 starters reappear with the addition of Pikachu
Choosing a Nickname
With your starting Pokemon caught, you will be able to choose your Nickname for your avatar.
Having done all that, you are free to wander the world and start exploring to find more Pokemon.