Hi i have a mac OS X and i want to put a password on it so that when i
switch it on it would not let me in unless i type in the password, how
should i do that?

I have a Mac on OSX, and I think all you need to do is the following:

1.  Open "System Preferences" (It should be in your dock at the bottom
of the screen -- it looks like a light switch).

2.  Click on "Accounts" under the System sub-menu.

3.  On the "Users" tab, un-check the "Log in automatically as (user)."
 Then press the lock button at the bottom to prevent anyone else from
starting up in your account.

When you restart, it will have your account on a log-in screen. 
Double-click your name, and it will ask for your password.

If you ever want to reverse it, go back to Accounts and choose "Set
Auto Login" and it will ask for your password to do so.

Hope that helps!

Answer: Use an OpenFirmware Password.  Note: This is the most secure,
and most hardcore method of doing this.  At the same time I highly
recommend this, I also warn two things, first... do NOT forget the
password.  Write it down somewhere, or use a word you will NEVER
forget.  There are ways of recovering the password, but it is much
harder than popping a CD in.  Second, follow the directions exactly as
they are.  Do not stray, or type funny commands in open firmware. 
This can kill your Mac.  =)  Now that we're past the disclaimer, read
on!

1) Boot your computer into OpenFirmware while holding (Command +
Option + O + F)

2) At the prompt type 'password' without quotes.  You will be prompted
to enter your password, and then to confirm your password.  If
successful it will tell you, and your password will be set.

3) Now at the prompt type 'setenv security-mode full' without quotes. 
This will enable the full security mode.  The macintosh will now not
allow you to boot it without typing in the firmware password first.

4) At the prompt type 'reset-all' without the quotes.  Your computer
will restart and you will be protected.

NOTE: In the future if you want to remove this protection, simply boot
back into Open Firmware like in step #1 and enter 'setenv
security-mode none', and then 'reset-all' and all the protection will
be removed.  If you're interested in internet resources regarding this
search on google for "Mac Open Firmware Password" and you'll find a
lot of great resources on this topic.  =)

Re tablerock-ga: Except I can simply boot off any Mac OS-X CD and
reset the password.  That is definitely not the way to go.  =)  Look
above.  However... in a simple home environment with just children and
the occasional friend coming over, that method will suffice just
great.  =)

 - Insolence