I was reading up on cron jobs and am not sure where exactly we need to set this up? Which file?

I put entries in the /etc/crontab file, will that work?

A simple command like rm -rf <filename> gets executed in the above crontab file.

However, a command like
"10 4 * * * root find /usr/local/phoneweb/logs -name "pw_metricsfile*" -mtime +7 -exec rm -f {} \;" does not delete the files even after 10 days.

Any help will be appreciated. Thanks!

that should be fine, but the syntax of the actual command to run get's confused easily. personally i would make a seperate script file with a useful name, i'd guess something like /var/scripts/rm_old_phoneweb_logs.sh, and just call that script on it's own from the crontab file. just make that script executable and you'll get a better level of clarity an abstraction in your crontab as it can just get really messy anyway.

You typically need to do
crontab -e
to edit the crontab settings for your user.
You can also do:
crontab some_file
where some_file contains your crontab settings.
The /etc/crontab* stuff is done for system-wide cron jobs (ie logwatch & logrotate). Only root can probably use those. But root can have his own crontab settings too.

Crontab settings (per user, not for the entire system) are typically stored in /var/spool/cron or something like that. On my FC3 box, /var/spool/cron contains a file with the cron settings from one specific user. The file has the same name as the user.