Press and hold to do a different amount, quick tap will zap your default amount, which is set at 100 automatically. However, you can change your default zap amount in settings. I found choosing a default amount you are happy with until you build up some sats and get the hang of things. I'm still learning myself tbf

you can set a custom zap amount in your settings, or you can long-press the lightning bolt to set a one-time custom zap amount

this is all covered in the FAQs