You need to have WinRar, 7-Zip or some other file archiver that can open RAR-files (as they are most commonly used for FoF songs) to add songs to FoF. This tutorial will teach you how to use WinRar if you are new to the whole process of adding songs.
WinRAR is a powerful tool that can extract and create both RAR and ZIP files. This tutorial will teach you how to use it.
Ok, you downloaded you first song, an you see a file that looks like a stack of books and says either RAR file, or ZIP file. The first thing you need is to download and install WinRAR.
After installing WinRAR, go back to the RAR-file you downloaded. It will be called WinRAR Archive. Double click it to open it. Go to the top of the program and click "Extract To". Locate your Frets on Fire Songs Folder (e.g. C:\Games\Frets on Fire\Data\Songs) and extract it there. Hooray! You just added your first song to FoF. Now you can open FoF and play it.
To create a RAR-file of your own custom song, right click on the song folder with all the files (guitar.ogg, notes.mid, song.ini and possible other files) in it. Click on "Add to "foldernamehere"". WinRAR will then create a RAR-file that you can upload to file hosting service such as Mediafire.