>>23405856First off, ask your parents if its okay that you have one.
Then, you should head to a Pokemon Center and ask if you can take the Introduction to Pokemon Care course. This will make sure you're able to take care of the Pokemon you get!
Once you get certified (keep the ID card they issue with you at all times!) you need to find a Trainer willing to help you catch a Pokemon. Try and look for ones that have earned many badges, not only because their Pokemon are generally stronger, but because they've generally wisened up and are more accomodating of your mistakes.
Get yourself at least two Pokeballs, in case one fails. Find a Pokemon you want, let the trainer do their work, and catch it when they tell you it's okay to.
Hope this helped!