How do you catch Pokemon?

Go into the tall grass, find a Pokemon you like, Get it weak, throw a Pokeball!!!!!
First you weaken it with the Pokemon you already have. When it has low health, throw a Pokeball at it and you have a chance of catching it. The lower its health is, the higher chance you will catch it.

You can also try putting it to sleep or any other special effects to make it even easier to catch