You will need to to read the manual on how to use the OPEN, WRITELINE, and CLOSE commands. Or read the FAQ on how to use UDFs and use one of the pre-written logging functions from the list of links provided in this thread.
Searching on Mapdrive will give you many hits, but this link has an example of what you want...
http://www.kixtart.org/ubbthreads/showflat.php?Cat=&Board=UBB1&Number=129914
