Hiding diapers under your clothes can be pretty easy as you can wear bigger pants or wear some boxes over the diapers to help conceal the diaper.
Wearing a snap shirt that snaps in the crotch can also hold the diaper in place as well as keep the diaper from being seen when your shirt happens to ride up.
You still might have a diaper bulge in the back or front that would be seen by you but other people around you that don't know you wear diapers would not really think about it even if they saw it.
If you want to hide diapers say from relatives, brothers, sisters or parents in your home then you could place the diapers in a backpack or in a box hidden in your closet.
If you need to wear diapers for medical issues or for bed wetting then you should tell your parents if you can so that you do not have to hide your diapers.