Here's a handy article that you may want to read
With This Ring
I review adult products, but as a female I can only point you in the direction of some highly rated ones I found with a quick search since I have no personal experience with cock rings and none of my partners have ever used one with me.
For starting, silicone seems like a good choice since it will be stretchy/have some give as well as not costing a large amount of money
Adonis Silicone Rings caesar - Cock rings - EdenFantasys
Once you have more experience with them, I know a few men who really enjoy metal cock rings. This one has been highly rated and looks rather nice
Tribal stainless steel cock ring - Cock rings - EdenFantasys