There aren't many options for 13 year old's to earn money online because of legal contracts etc.
You could start a youtube channel though and make interesting videos and then have your parents sign up for Adsense for youtube.
Your parents or some other adult would have to sign up for Adsense because if you're under 18 you can't have an adsense account.
You could also make websites and write content if you're good at it and then once you get website traffic you can place the adsense code on the website and earn money with it that way.