For soft materials take 3/16”-5/16” of the measurement and that will make a good starting point. Since most of the rings have an unstretched diameter/ circumference listed use that as a base line. It is always better to be loose than to tight.
For hard materials it is advisable to go with your measurement or slightly bigger. Remember that your cock is going to get bigger with it on. Once again, bigger is safer.
Now let’s look at materials. Just about every material used to make sex toys can be found in C-ring and then some. About the only material I have not seen is glass but I am sure they are around. (haha get it?) For beginners I highly recommend going with a soft material like jelly or TPR/TPE. This allows you to become comfortable with wearing them, plus they are pretty cheap. There are a bunch of rings that are designed for “One time use”, these typically are under $10 and last one or two uses. That way if you find you do not care for them it is no big loss. As you get comfortable with them you can try different materials.
Something to think about, some materials like jelly and TPR may grab onto pubic hairs, you can do some manscpaing or use a bit more lube to keep it to a minimum.
I would hold off on the harder materials, silicone, metal, stone and wood until you are experienced with cock rings and know your size
Consider this, if for some reason you start to experience a lot of pain and that ring is so tight it will not release any blood, how are you going to get it off? This is even more important when you wear it around the whole package. Ice may work or even thinking about doing yard work but that is no guaranty. Silicone can be cut if you have a sharp knife or large scissors; stone could be broke maybe with a hammer but metal??? Well you get the picture, none of these options would be very fun and chances are you will need to go to the ER to have it removed. FYI, the ER will have a heck of a time with Stainless, it is very hard and not easily cut. I’ll save you the details but there is ice and sparks involved.