I have a wall plug in my bathroom now it seems to be the only thing on this line when I turn off the breaker, can I change the standard plug to a gfi and then jump off the gfi to a on/off switch then to a shower light?

Yes, the GFcI will come with directions on how to wire the additional device, down line from the GFI , GFI will come with taped terminals that are used for down line protection,
I'm pretty sure this will do fine.

I am no electrician, but I have had some experience

Agree, you can make everything after a GFCI protected but read the yellow label that wraps around the unit to be sure you do it right, otherwise it may work but not be protected