There are a few casting subreddits, including /r/metalcasting. /r/silversmithing might be able to point you in the right direction.

Be aware there are different methods that work substantially differently and require different tools. Lost-wax casting (including lost-PLA and lost-resin) is probably the most common, but has the highest entry costs. The exact tools depend on whether you do centrifuge or vacuum casting. Sand casting is a smaller barrier to entry, but is more limited in the complexity you can achieve. Cuttlefish casting is also quite low, though is also limited (but you get an awesome texture on your casts). Which of those two is better depends on how easily you can get cuttlefish bones (I have heard some people get them from restaurants) and whether you want to carve or cast existing objects.

There are probably other techniques, but those are the ones I know. I mention the difference because you will probably want to focus on one at first. Be aware that certain techniques can be dangerous, especially centrifuge casting (metal that is almost 2000 degrees tends to do that), so if you can find formal instruction, I would recommend that.