I have a directory called 'test' which contains many junk files. I want to automate the clean-up task by removing all files which are older than one month. What would be the best option?
Run a find command with the appropriate switched (see man find) and put the command in the crontab file. Make sure you carefully test first.......
... or try to find "tmpwatch" software ...
I'd suggest creating a macro (eg: clean_testdir).
Then as either root or the user which you are logged on as, do ...
crontab -e
This will open the either a copy of the existing crontab file for root/applicable user or a new file.
If you wish the macro to be run every night then you may do the following ...
45 23 * * * [location+name of macro mentioned above] > /dev/null 2>&1
This will run the macro at 23:45 every evening.
Hope that helps some. 
I've just become familar with crontab is the past week
Cheers,
Cameron.