Once you've learned to count teeth, find the steepest and longest hill you want to be able to ride up and then find the gear combination that lets you ride up at a reasonable cadence (crankset rpm). And climb that hill when you are tired.
This will help you understand what your particular gearing needs are. It is not possible for someone on the net to tell you what you should buy without knowing what you need.
It is not necessary to change the cassette when changing the crankset.