First thing you're gonna wanna do is grab an ISO or GCN file of the game. I am unable to link to any due to smashboards rules, but you can google around for torrents and stuff.
Alternatively you can download SleepyK's prehacked ISO, which is how I got started. You send an Email to : " sreeepyk@gmail.com " (yes, three e's)
and an auto reply bot will send you a response, with some very useful links for ISO's and stuff.
Texture replacements for both stages and characters are pretty easy. The only problem is that textures for characters are built for certain costumes. This means the White Power Ranger Captain Falcon can only replace the white outfit, and so on.
Music hacking is a good bit more complicated but we can deal with that later.
Check out the main thread :
http://www.smashboards.com/showthread.php?t=247119
you're gonna have to download some programs like GC Rebuilder and/or GCTool.
Also, SleepyK has tutorials on youtube for most everything, though a tad unorganized.
Feel free to ask me any more questions if you run into any problems, or post in the main hack thread : )