It updates your dynamic IP to no-ip's central DNS server.
Putting it in your rc.network file would probably work best, but putting it in rc.local would be sufficient. You don't need sudo because startup commands are run as root anyways.
I'm not sure if you've tried any of that, because you were extremely vague. But if you have and it doesn't work, make sure you put the full path to the program (/usr/local/bin/noip2 or whatever), not just "noip2". Also make sure that noip2 goes straight into daemon mode, or figure out which flags you need for that, otherwise it'll slow down or lock up your boot.
Incidentally, ESR's
"How To Ask Questions The Smart Way" might be something you should consider reading. Particularily the point, "Be precise and informative about your problem":
* Describe the research you did to try and understand the problem before you asked the question.
* Describe the diagnostic steps you took to try and pin down the problem yourself before you
(I realize that it's not a bug, but those still apply)