That's because ROMS are pretty much illegal, so technically we cannot publicly send you any link. But you can google for "NES ROMS" and "NES emulators" and you will get all that you need.
Most emulators also come with a convenient readme file that tells you everything you need to know. In case you should still need help, write here. Explaining how to use an emulator is not illegal.
Generally, you need to choose "open ROM" from the File menu, then specify the path of the ROM file. Also you usually can configure a gamepad, if you have one. Instructions may vary depending on the emulator you choose to use, though.