edit: colucix has a better overall response...
A cron job is simply any executable file/program added as a cron entry. Do "man cron" and "man crontab" for more information on how to add the entry in your specific distro.
In this case, nothing here looks like you would need more than basic shell scripting. To see if grep found something, check it's return code. In shell script, $? is the variable that holds the return value of the last executed command.
Code:
example:
grep "fish" /tmp/*
RET=$?
For the e-mail part, you need a command-line e-mail program. mailx is good for that. If you need to send through remote smtp (with passwords and stuff), there's a program called sendEmail which I've used before that is also good. There are a lot of script examples out there. Search this forum for shell scripting.
Also, you should probably list the distro you are using. That might help people give you more precise advice.