Parents sometimes think it's cute to have younger children babysit their kids (I first started when I was like twelve or so) but it helps if they know you well, like if you know their family from church or something.
To get "known", I would just suggest babysitting for families you know first. Tell your parents that you want to earn some money doing it, and to spread the word, and soon enough you should have a couple of invites. If you do a good job, they'll inform others about it, and soon you'll be all over town watching kids.
It might be hard for you, though; babysitting any kid older than six or so would just be bad (too close of an age gap), and most parents want to keep their younger children (0-3 years) around them at all times, so the age range you could responsibly babysit is kind of small.