You can drink things that act as a diuretic such as tea and coffee and take muscle relaxers that will relax your bladder muscles and cause you to wet yourself or your diaper without you being able to hold it back.
If you want to wet the bed at night you can also drink plenty of water or something else that will fill up your bladder and then as you fall to sleep place your hand in a bowl of warm water.
I heard that having your hand placed in a bowl of warm water while you're asleep can make the person wet the bed and I tried it on myself and it caused me to wet the bed or in my case wet my diaper without trying.
That's a good way to fake bed wetting if you want to get put into diapers or get your parents to buy you diapers for bed wetting.