My shower only diverter is a Moen, (model unknown). It is a turn only type. In the last couple of days I have noticed the knob is increasingly difficult to turn. Today if feels like if I force it it may break. Is this an easy fix? Or do I need to call a plumber? :( I'm not that handy but not afraid to try.

In general, it means that the packing nut is leaking and there is green stuff on the stem.

Could be as easy as just tighening the nut and cleaning the stem. You may have to replace the packing (washer in modern faucets).

While you at it replace the washer at the bottom of the valve.

Steps are:
1) turn off water, drain and make sure it's off
2) Turn valve all the way on
3) take off knob
4) clean with brush and possible 400 grit sandpaper
5) Try tightening packing nut
6) Close valve, turn on water and see if it leaks when open and closed.

End of part #1.

If it fails to work:

Turn off water as before and turn valve fully on. Loosen packing nut and continue turning knob on. Assembyly should come out.

If you have a nice hardware store, they may offer to find and replace the parts free.

Worst case, you have to order specific parts from a plumbing supply house.

Re-assemble in reverse.

Major potential problems:
1. No or leaking shutoffs for the shower where replacement is necessary.