how do i have to calculate the value of the input/ output capacitor. the datasheet only specifies the kind and suggests a value.
Don't. Just read the datasheet carefully about what types of capacitors to choose, where to place them, and roughly what values you need. Then stick to that. If you want to be extra safe, double or triple a value. For 99,99% of applications that's fine & there's no point in attempting to do better than the engineers who designed the IC.
People that have problems with voltage regulators, are the folks who think they know better & leave out capacitor(s) that should have been there, or use ridiculously small or ridiculously large capacitors in the wrong place. Or wrong types. Just read carefully, follow datasheet example & you'll do fine.
Btw. large buffer capacitors on the
input (not the 100nF-1uF ceramics you often see there) depend on current you're drawing, transformer properties, allowed ripple voltage, the room you have between input voltage & what regulator needs as minimum to maintain output voltage, etc. If in doubt there, post schematic + your exact requirements & people can tell you if you're on the right track.