With Mozilla, (Firefox)...all you have to do is double left click on the star (it's on top of page, next to your search bar),then add a tag if you wish, or just click "done"...and it will be added.
As for your folders, that depends on your mail service..(MSN,Yahoo, etc.)...they are all a bit different, but generally when you open your mail there should be a place where you can click on it to add a new folder, or add something to an existing folder..click around and you should find it.