I have this inst. audio file for a certain song
but the pitch/key is too high for me to sing.

How do I lower the pitch/key down for this inst. mp3 file?

Any ideas? Sorry that I'm a completely noob...  

Import the audio track, select from menu Process-Transpose (make sure transpose audio box is checked), and specify the transpose key amount.

Edit : also select the entire clip first (or parts to be transposed).

I would use V-vocal or if you have a 2r party plugin like Melodyne, then i would use that..

CJ

I'd vote for Process > Transpose IF you don't demand real time processing. It uses an offline algorithm that sounds very good with modest amounts of strettching. Sometimes I record background vocals listening to the track a semitone down, and when it gets transposed back up, the vocals have a slightly brighter timbre that works well.

i like andertons idea,i do the same thing theres always been something about a recording something lower then bringing it back up that brings out a little punch in the upper register.
i used to do it back in the analog days and still do it now.