question1:
how do i set so my crob jobs does not auto send all the results to my email?

i remember its something like

* * * * * * /command/ >>dev/null

is this correct?

at the moment, cron keeps sending me output of the job which is a few MB. I dont want to receive this.

question2:
I want to run some programs using cron, I know you can run th program every minute by using
* * * * * command
and to run it at every hour u use
0 * * * * * command

but is it possible to make it run every half hour? or every 15 minutes?

thanks in advance

That should work.