This is probably covered in the manuals but I have not found it yet. I need to make an MP3 file that is the concatenation of 2 smaller existing file, could someone tell how I could do this ? I am totally new to Audacity so I am a greenhorn. 
Thanks,
Ray Z
Are the originals MP3? If they are, you’re new MP3 is going to have higher compression sound damage than the originals. Audacity makes a whole new one, it doesn’t manage the old ones. It sounds like you’re looking for one of the pure MP3 editors. They can do limited editing tricks without taking the performances apart and putting them back together again.
Scroll down.
http://wiki.audacityteam.org/wiki/MP3#re-encode
Koz