Practically speaking most check the wikihttp://datacrystal.romhacking.net/wiki/Donkey_Kong_Country
Such pages are just the overview so make sure to select the ROM map, RAM map or whatever else as appropriate ( http://datacrystal.romhacking.net/wiki/Donkey_Kong_Country:ROM_map
More seriously graphics is often the first step for people in hacking.
Basic thing is open game in tile editor (I like crystaltile2, tileggd and tiled2002 but others pick other things). Select likely option (most tile editors name their mode selection for the consoles they are working on, tileggd on my list above being the main exception), press page down/scroll down a lot, select other likely option (some consoles have multiple modes), press down a lot again.
If you are getting to grips with assembly then you have other options. It is for the GBA but the logic is the same for almost everythinghttps://www.romhacking.net/documents/361/
Before that many emulators will have sprite/oam/bg viewers. If you just want to rip things then these are good, you might also try dumping things from that and searching the ROM for it.
Some systems like the GBA will have compression. Emulators can log calls to things and you can look at those, or some tools will scan for the fingerprints of compression and attempt to extract it.
Newer systems also have known formats shared between many games, and in some cases even formats a decent image editor can handle.
Music. Varies dramatically between consoles.
Again check wikis, if you are working with an arcade title then MAME is worth looking at (there are usually so few games for a given board that the MAME/MESS devs often documented the ROM by default).
The GBA and newer as well as most things based on CDs or with file systems (so the Amiga for instance) often use either formats known to game hackers, conventional audio formats or in the case of CDs then CD audio. Most of these formats have been documented and tools to convert made. Encountering a new one tends to involve learning a bit about how audio formats are made ( http://soundfile.sapp.org/doc/WaveFormat/ https://fileinfo.com/filetypes/audio https://wiki.multimedia.cx/index.php?title=Category:Game_Formats
) and knowing the hardware -- the DS for instance features the ability to decode ADPCM audio so consider that as likely to come up, Microsoft stuck a decoder chip for some of their WMA style audio formats in the 360).
16 bit mainline consoles and older. Custom from the ground up, and many of the peeps doing audio were good programmers in their own right. Maybe shared format between games from the same devs/publisher. Equally most things will not be wave type audio, instead sequenced (think midi) audio for reasons of space and playback capability. Where wave type stuff does exist it tends to be very short.
Good thing you have got to grips with ASM as hacking audio for unknown games on older systems tends to involve a lot of it, in the case of the megadrive/genesis it even had a Z80 CPU governing the two audio chips and able to run its own code.