I would like to know who worked for it, so the dialog will give credit
Maybe Stephane Thomas? Mabe Mario Filice, Paul G? Paul W? It is likely no one knows at this point who provided the current Italian translation so please just list your name for now. If the original author contacts us or someone can track down the old commit, we will be sure to give him/her proper accreditation.
How do I do it in Github?
If using Windows, the easiest way is to fork to your own branch here: https://github.com/LMMS/lmms/fork, then download the GitHub client and add, commit, sync. Once that's done you'll have to issue a pull request on the github lmms page to get them merged. This may sound like a lot for a few translation files so if you can't figure it out after a few tries, email them to me and I'd be happy to push them in for you.
Do I have to put both it.ts and it.qm?
Yes, please provide both files.