different type of resin for watersofteners?

I removed all the old resin from my watersoftener and looking to buy new.
Are there different kinds of resin beads to look for?

My tank size is 8" wide and 40" high.

How much resin do I need?
Do I need to put a bed of gravel in first?

Any advise is aprreciated

thanks

Ron

I answered this in the original post.