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

Go to settings to define the default amount And you can always hold for a second the zap button to specify a custom amount

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