I tried every website to download a rom and they al did not work and now i dont know what to do
Don’t download games, that is illegal and not supported by Citra for obvious reasons. Also those game tend to be broken and/or packed with viruses and malware.
Just buy the game in the Nintendo e-Shop and follow this guide.
Or if you want to save some money, get the game cart second hand and follow this guide
It’s very easy, really. No need to pirate.