How can I tighten this screw? It's loose and preventing me from being able to rotate the knob and turn the water on and off. I can't pull the handle out enough to get a screwdriver in it and tighten? I'm including a picture to hopefully clarify what I'm asking. (Apologies in advance if I'm not using proper terminology as I'm not very good at this stuff.)

You should be able to pry up the piece in the center of the plastic handle, usually has a logo. Under that, there is a screw that holds the handle in place. Remove the screw and handle and then you can access the screw in question.

Thanks so much Dennis. I'll try that tonight!