i want to reset my computer to close everything and start new

Throw away any files which may contain sensitive data, securely empty the Trash, insert its original disk, restart the computer with the C key held down, use the Disk Utility to erase the partition with Mac OS X installed, and install a fresh OS onto it.

If you're going to sell or give away the computer, include the disk with it.

(106763)

http://support.apple.com/kb/ht1544

Boot from your Leopard installer DVD. Navigate to the selection of the target disk. Click once on that disk, then click on the Options button. Select the Erase and Install option, then click on the OK button. Continue with the installation procedure.