Hi Tracey.
One of the simplest methods of importing existing music files into iTunes is to open iTunes, and use the File | Import... menu option. You can then browse to where your music is currently stored, and select the entire folder, or individual files. iTunes will then inspect these files, copy them into the iTunes library, and then report any problems or issues encountered.
Can you tell us where the process of getting your existing MP3s into iTunes seems to be falling down?