PC games or console stuff? In some cases there is not a lot of difference but in others there is a reason they don't mix too often.
For older console stuff then
http://www.romhacking.net/ is a good start.
For anything newer then this site has a fair bit. I mostly stick to the GBA and DS stuff that does not have a dedicated site/following* (my main guide to it all in my signature) but there are plenty doing 3ds, Wii, Wii U and switch stuff on the Nintendo front, and you will probably find someone to talk to about xbox or playstation stuff as well. Not sure where to point you for IOS/android stuff at this point.
*certain games almost inevitably get an involved hacking scene. This would be things like Sonic, Mario platformers, mario kart, pokemon, advance wars, final fantasy to some extent, dragon quest, chrono trigger once had a big following, fire emblem, fire pro wrestling somewhat surprisingly.
For PC stuff then most start with mod making and branch out from there. Eventually you find the tools devs provide to be less than what you want to do and figure out what to do from there. Those that don't start with game modding tend to start with cheat making.
I doubt instant messengers like discord will be that good (granted I think discord is bad anyway -- it is a proprietary protocol and you can't run your own server, such things are then almost doomed to fail), and things like reddit are not designed for long form discussions where ROM hacks can go years without something happening, and then someone can wander in cold and reinvigorate things with a single post. Such a thing is not even uncommon. There is a high barrier to entry to a lot of this -- you are going to end up with some fairly out there skills in computers, file formats, programming, some aspects of maths/logic, patience is useful to have and so is a reasonable bit of determination/resilience so if you decide to write somewhere off after the first insult then you have failed and were never likely to make it.
https://docs.google.com/document/d/1iNSQIyNpVGHeak6isbP6AHdHD50gs8MNXF1GCf08efg/pub?embedded=true
http://hciweb.usask.ca/uploads/332-aim-assist-cameraReady-v8-final.pdf
http://wiki.xentax.com/index.php/Game_File_Format_Central
http://fileformats.archiveteam.org/wiki/Game_data_files
https://en.wikibooks.org/wiki/Guide_to_Game_Development/Data_file_types
Some programming skills will help here, especially for PC which you can readily debug.
Ultimately though it kind of boils down to "computing - learn it"; if you want to hack an MMO type game you will probably benefit from server making skills and databases where hacking a more conventional game's 3d graphics is a different matter entirely. By all means specialise a bit, and find which fields of hacking do it for you (some like translation, some like making trainers/cheats, some like making new levels, some like fixing games...), but most ROM hackers and high end game modders have fairly in depth technical knowledge of a fair few quite different fields, ones not necessarily taught all the time in schools either.
