how to automatically send mail to another person at a certain time

You can use CRON - Do you know what cron is?

Yes, try CRON. See this page for more info http://www.linuxhelp.net/guides/cron/

but mi not on linux. me on windows

You can use the Scheduler in Windows XP, go to Start -> Control Panel -> Scheduled Tasks.
Click on Add Scheduled Task and there will be a wizard to guide you through the process. Hope that helps!

i try that before but then i want to ask is that the task is suppose to open which file or wat program because i try running the mail.php but in the end it did not send, it just open the file at the set time

You're going to need to have to have PHP installed (alongside Apache or IIS). Schedule your task to open your browser (IE, Firefox, Mozilla, whatever) and having your broswer open your PHP file. You URL will be something likt http://localhost/mypage.php

When the browser opens the page, the code will be parsed by PHP, and thus your mail should send.

Hope that helps.

how are you going to let the browser auto open the page?

I'm thinking he can make a shortcut URL, and have the task point to it. So that will open the browser at the given adress.