roms are illegal so i wont tell you where to get them. but i will explain the operation of emulating them.
whatever emulator you have and whatever system it emulates i dont know. First you need to know the file types:
snes: smc, sfc, fig, and probably some im forgeting.
nes: nes,fmc
n64: u64, v64, ect.
Obviously you need the emulator of the correct system. i reccomend
snes: zsnes, snes9x
nes: nesticle, fce ultra, (everyone has there own prefrence)
n64: project64, mupen64, UltraHLE 2064
als you do is load the rom to the emulator (file/load/..) and it should run.
n64 emulators require plugins.