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As someone fairly new to chastity play myself I would be hesitant to make a strong recommendation or provide a lot of tips, but you should absolutely look at, or ask around in r/chastity or r/chastitytraining.
I know a lot of the prevailing wisdom seems to be to try inexpensive cages from amazon or other online stores to find what you like and then look at buying a nicer version of that. I'm currently using a knockoff holy trainer which is by far my favorite of the cheap cages that I've tried out but your mileage may vary.
From my experience, it's very true what they say about wanting something that is going to fit snugly in order to prevent you from starting to get hard and then running out of room to grow, which can be really unpleasant.
I also second what the other commenter said about lube being your friend when putting on your cage. I prefer silicone lube(if you're using a cage that it can be used with) since it last longer than the water-based kind.
I'd strongly recommend trying it out for yourself before introducing it into your relationship, if at all possible. And if you are concerned about how she will take it, DON'T surprise her with it! Have a conversation first, and let her check out the cage and ask questions before she sees it on you.
Assuming that chastity is something she's into, you want to be ready with a cage that you are comfortable wearing right away and for some duration (a day at least). That takes practice, and might even take a few different cages until you find one that your junk is happy in.
I would recommend starting with cheap cages until you find the size and style that fit you best. I am a big fan of Holy Trainer cages, but you are better off to start with a cheap HT knockoff with multiple sized rings before spending $100+ on a chastity device.
For your first cage, I would go with some kind of hard plastic or resin. Silicone cages are too flexible and sticky, metal cages are really heavy on your junk. No need to stretch your scrotum more than necessary.
Sizing tips! Unless you have a donkey dick, start with the "small" size cages. The cage should roughly be the size of your totally flaccid penis. Many people think they need "room to grow", but this actually makes things less comfortable.
FYJENNICC New HT-V4 Chastity Device Cage Male Briefs Men's Underwear 5Rings Transparent (Small)