Pick out one of your favorite songs on the radio. Pop songs are GREAT for back up vocals. Try to only sing what the back-up people are singing. Try to figure out if they are singing the Melody or the Harmony. The Melody is the main theme of the song, and the easiest line to pick out. The harmonies are a little more challenging to hear. 9 time out of ten, the back-up singers are singing Harmony. Here are two songs that are great to pick harmonies out of. I picked out two duets because you can hear the voices very clear in these songs.
For instance:
Try The song "Lucky" by Colbie Callait and Jason Mraz. If you are a female, sing her part. If you are male Sing his part. Both of them switch off taking the harmony.
Another good song to try Harmonies to is "Whiskey Lullaby" by Brad Paisley and Allison Krauss. This song has amazing BEAUTIFUL harmones. Girls follow Allison of course, and guys follow Brad.
When you sing along with these Try turning the Music down a little so you can hear yourself CLEARLY over the music. Plug one ear and try to follow it.
Don't be discouraged if you don't get it down the first couple of tries. This does take time, Developing the inner ear, and of course...PRACTICE. If you need help, just ask!
-Melody