It is best to scan it in (make sure it is not copyright protected, or is something you created or got written permission to use). Scan it at high resolution, then 'place' it in Illustrator, hit 'live trace' and it will create a vector graphic, which you can edit. It will give you choices of how you want to trace it ('detailed drawing', 'black and white drawing', etc.); you may have to experiment. Logos should always be in vector format so you can size them easily and they won't be compromised or get pixellated.