Hi there transfemandgay!
While I haven't personally used one, I know a good deal about them!
This article has some of the basics, but I can sum them up pretty well:
1.
Start out with something adjustable/stretchy. There are lots of options available that are adjustable (a looped piece of silicone that can be adjusted in size with a placeholder) or very stretchy. They often sell them in multi-packs so that you'll get the right fit no matter what. They can be placed either just at the base of the penis or behind the balls as well to give a more intense squeeze.
2.
Give yourself a (not-so)dry run. If you'll be using it for partnered sex, don't wait until The Moment to try it on!
Practice putting it on when soft, getting hard, and then taking it off to make sure that it fits comfortably and that you actually enjoy how it feels. Adding a bit of lube to it when you're first putting it on will make it a lot more comfortable. Tips for a correct fit: the penis might feel a little more stiff or look a little bigger than normal, but it definitely should NOT hurt or feel numb or look a lot darker than normal. If that happens, you should definitely take it off, because your circulation is being cut off almost entirely.
3.
Keep an eye on time. Most of the literature says that you shouldn't leave a cock ring on for more than 20-30 minutes, just because the tissue needs to be able to have blood flow through it to stay healthy and work properly. So you're right - a metal ring that wasn't fitting right or didn't feel good might be harder to remove without assistance than an easily-snippable silicone one. But if you start with a few silicone ones, you may get a better sense for what size/tightness works best for you, so that you could graduate up to something made out of a less flexible material.
As for buying them, that's a little bit trickier, as most sex shops (in person and online) are 18+. Are there any older folks in your life that you trust that might be able to help you out?
Another alternative is to go DIY/low-budget: I've heard from some young folks that the ring at the bottom of a condom can have some of the same effect, and is really easy to procure/snip if needed. Just cut off the top of the condom and there you go!