Fonts can be opened with Font Book - application. If you have 10.3, it is located in your Applications folder.
You can see the charachter map on the menu bar. Go to System Preferences, in there to International > Input Menu. Select in that all the layouts you want to use (e.g. US, Italian PRO), and select "Charachter Palette". Then you will see a small flag on your menubar to indicate your keyboard layout; scrolling down from the flag you will see charachter palette, and if you clikc on that it opens a new window.
Furthermore, many letters like ãñäõéáóèàò etc have a fast shortcut to be done, alt-something, so alt-u and then the letter for üäö etc, alt-n for ñõã, alt-e for éáó, alt-` for àèò and so on.