Go to the forum of your choice. In your case, I'd suggest the Mobi Books section. On the left-hand side, immediately above the blue bar with various thread options, there is an inconspicuous blue box with "start new thread". Click it. Don't forget to add a thread title (author surname, comma, firstname then colon: Title, , v.1, date.) or to upload the attachment. (Believe me, I've forgotten.)
It's not a dumb question at all. We all have to start somewhere, and you should have seen the help I needed when I was starting out.
Of making many books there is no end [Ecclesiastes, 12.12]