Isabel,

"When I'm playing along with a CD ..... the instrument I'm playing doesn't sound right" - might be the fault of the CD player (I'm not joking, I can tune exactly to my Hi-Fi CD player, then play along with my CD-Walkman and I'm off-key).

Electronic tuners can vary too - if you're playing with other stringed instruments and using a tuner, it's best if you all use the same one.

It really is all down to practice and a little understanding of the physics involved. Keep at it and your ear will 'sharpen-up'. Once you can hear the intervals between strings accurately, and detect the slight wavering 'cycle' when strings aren't resonating perfectly together you'll be on your way.

Johnny.