If you want to tell your mom that you want to wear diapers at the age of 13 I would recommend sitting down with her alone such as in your room or her room and then start by saying that you love to wear diapers and see what she says.
Tell her that you enjoy wearing diapers and want to wear diapers part time or full time depending on your preference for the diapers.
Explain to her that there are other people out there your age and older and younger that also love to wear and use diapers.
You can even show her this website if you want too.
Other than that I personally would just wear the diapers in private and not even tell her as it could go badly or could go a good way.
I had thoughts of telling my parents or mom when I was a kid but then decided against it and I think that was a good decision.
But it's all up to you on whether or not you tell your parents.