I got a job at my neighbor's when I started at 13. They saw me with other kids in the neighborhood.
Now I guess there are classes you can take in babysitting. If you have no experience with siblings that is recommended.
At that age, you defeinilty can start babysitting. My suggestion is asking a mom with a 2-4 year old at home if she needs help. Offer to watch her children while she runs small quick errands, or play with the kids even while she is home trying to get other work done. This wil get her to have confidence in your abilities and she will start leaving them from longer, etc
There is a website (care.com) where I found the family I nanny for, although at only 13, not too many are likely to hire you if they don't already know you. Best bet for you is to ask around and see if you personally know any families who either need a babysitter or know a family who does. Good luck!
I'm 13 and I really want to start babysitting, but I have no idea how to get a job... Any ideas are appreciated!! Thanks!