Change Screen Resolution in Debian

Hello,
I'm currently running Debian, with Xfree86 on it, and cannot change my screen resolution. When i had installed it, i did not select the right resolution, and am wondering how to change this. I've also heard that you can hit Ctrl+Alt+ the (+,-) arows on the key board, to change the resolution, but I do not know how and when to do that.
Can anyone help? Please.
Thanks :)

Edit
If anyone cannot help me on this, then can anyone give me steps to reconfigure my xfree86, so it can make the resolution higher. It is currently set at, 640*480.
Thanks.

Try this command as root:

dpkg-reconfigure xserver-xorg

Note that most Linux distributions (including Debian) set the default resolution during the initial xserver configuration which often occurs during the OS install. This may save you trouble in the future.

The command given to you by doncrawley should reinitialize that configuration, so go ahead and do that.