Look at it one way, once the vocals are out, you just have to adjust your octave. The music will be in the same key, but as long as you know how to adjust your voice any song can be sung lower. If you're really having trouble, look to songs sung by men. A man singing in the tenor range is easily translated over to the alto range for women. :3 That's probably the easiest way to go if you don't have any music reading/training experience.