This is a good basic look: https://www.bitofbritain.com/The-Stud-Book-p/0855.htm
There have been good articles in Practical Horseman and the USEA magazine, too, but I have no idea which ones. And your best resource is probably your trainer if you take lessons with a past or present UL eventer (hopefully you do).
When is as soon as you need them-- pretty early on if you have a horse with four shoes and do dressage/ showjumping on the grass and the ground gets hard. Even at BN and N it’s nice to have the option of studs at least behind. At minimum you should buy a rubber safety tap, a stud hole brush (wood handle with a pointy end and a brush end) WD-40, a magnetic dish, a plastic tackle box with lots of little slots, a bigger toolbox or bag to keep everything in, some kind of stud plugs-- either rubber/foam or cotton, a good quality adjustable wrench, and the studs themselves. Any tack store that caters to eventing (VTO Saddlery, Bit of Britain, probably Smartpak) will have the tap, brush, plugs, and studs, and you can get the rest at Walmart or a hardware store.
What studs to get and what to use when is a much bigger decision and depends a lot on footing, level, and the horse itself as they definitely can have preferences.
https://www.vtosaddlery.com/product/NFS.htm I typically use the medium sized studs for most events. Of these I think it is stud O (the medium road stud) on the inside behind and often in front as well. Since it is blunt it makes it less likely the horse will stud itself. On the outside I typically use something like stud M (point/ grass tip) on good to hard ground-- this would be at Prelim and below, if I were showjumping on hard slick grass at Intermediate or higher I might use bigger versions of the same studs. On very wet, sloppy ground I would use stud K (bullet) or similar on the outsides on all four and probably on the inside front, and if I were running at a higher levels, I might use bigger versions of the same shaped studs. If I were hunting and expecting to ride on the road, or doing roadwork, I would use T or G (very small roads) all around.
Very generally I try not to use more stud than I need-- you want the horse to be able to “slip” a little since no slippage at all increases the strain on the tendons. You can always ask, even if your trainer isn’t there most other riders are happy to show you what they are using and make recommendations.
