N64 games don't use a file system like some of the DS and later devices. The devs just bundled everything together in one file and reference that internally.
To that end you get to figure out where something you want sits in the ROM. Variety of means of this depending upon what it is, and your skill levels, but this is generally what most of those learning to ROM hack documents cover.
As the N64 was generally a flop with no games then many of the bigger titles have been looked at, and might have surprisingly good tools for modifying them, documentation of their formats or at least existing hacks you can compare with the base game (if you have a translation then even if it is from a language you don't know to another one you can be fairly sure that what gets changed is mostly to do with the text, font and handling of both).