Before telling your mom that you want to wear diapers and act like a baby you should decide on how your parents may react.
Are your parents the understanding type or not?
If your parents or mom is the understanding type then telling your parents or mom may be a good thing but if they are not very understanding then it would be best to avoid telling your parents or mom that you want to be treated like a baby or act like a baby and wear diapers.
Your mom or parents are unlikely to treat you as a baby or put you in diapers but it could happen but don't be surprised if it does not.
If you want to tell your mom that you want to wear diapers and act like a baby 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.