I have a first year U11 goalie so just went through this steep learning curve over the last couple of years as well. As a player growing up I had no idea about any of this either.
Most has been covered here, just a couple of things to add.
Try on pads at a store to get sizing, as mentioned they all fit differently. My kid tried 28" Warriors and they were way too big but the 28" CCM's fit perfectly for example. You also need to try them on with skates on to get a proper fit. The biggest fit item is where their knee falls on the knee stack. Pads should also be fairly loose in the straps, the pad needs to rotate as they go down in butterfly. We found the 24" association pads were so tight even at their loosest that the pads didn't rotate at all, once we got our own 26" set this was much better.
Also, with modern pads you need to have knee pads like this
https://www.hockeymonkey.ca/accessor...gh-guards.html for protection but also fit. If you go try on pads without wearing these it's likely that they will all feel too big.
Once you know fit/brand you can keep an eye out for used stuff. I did find it really difficult to shop for used gear on marketplace/Kijiji, it becomes tedious driving all over to try things on. It also seems like the online marketplaces have kind of killed the market at the used stores, I found the selection at those to be terrible, lots of 15-20 year old stuff.
The rest of the gear is feel and fit as well, but a little more intuitive with sizing than pads I found.
