homebrew roms

Either knowledge of the N64 assembly language or knowledge of a language that can be compiled to N64 machine code. Based on the tone of your question, I would guess that you're not too familiar with programming in general. Writing for something like the N64 is rather difficult because it is an old system on which homebrew was unsupported. Good luck nonetheless.

If you wish to develop software for the Nintendo 64 (the Ultra 64 platform as it's commonly referred to as) then you must download a Software Development Kit. You can find these on bittorrent and across the internet - a Torrent search engine and uTorrent or Vuze will serve you well.

However, as mentioned, programming the N64 can be a bastard task to do - Though one person had suceeded in doing so without prior programming knowledge (find his rom on a torrent site, it's in one of the PD-roms collections. No source included) it does require some serious strength to pull off this sort of thing alone. (For me it took two years to fully learn C++ computer programming on my own... Wish i had better internet...)