It sounds as though you are a complete novice, so I shall try to go through this step by step. Turn on your PC and open the DVD drive by pressing the eject button. Place your copy of the Vista DVD on the tray and gently push the tray until the drive pulls it back in. At this point, depending upon how quickly you have done this or how quick the BIOS POST routine has been, you might need to restart your machine. After the post has completed, your machine will attempt to boot up. The normal boot order is: Floppy Drive (if available, not all machines have one), CD/DVD Drive, and Hard Drive (this is the order that I recommend, so if your Hard Drive is booting first then you need to change the boot order in your BIOS by refering to the motherboard manual and following the instructions). Once you have got the boot order sorted, when you start your PC with the Vista DVD in the drive, a message will appear on the screen saying 'Press any key to boot from CD...', at which point you need to do what it says otherwise after a short while (typically 10-30 seconds, but varying according to your BIOS), the system will move down the boot order and try booting from the next device in the list. At this point, I usually hit the space bar. Your system will now start to boot from the DVD. Follow the on-screen instructions.
When you boot from the Windows DVD, your first stop is the Install Windows screen. Choose your language preferences and click 'Next'. On the following screen, click 'Install Now'. The next screen prompts you to enter your product key. As your copy of Vista is more than likely an OEM copy, you will find a sticker with this key affixed to the case of your PC. The next screen asks you what sort of installation you want. Because you booted from the DVD, the Upgrade option is disabled. Click the 'Custom (Advanced)' option to continue with a clean install. The next screen, entitled 'Where do you want to install Windows?', display a list of the available drive(s) in which you can install Vista. Select the required drive and click 'Drive Options (Advanced)'. Select the partition and click 'Delete. With this option, you must respond to the confirmation requests before continuing. You can then click the 'Format' option and your disk will be formatted before continuing with the installation procedure. Note that there are actually 2 form of format here - quick and full. As the disk has been used before, select the quick option as it really is much quicker. The installation will continue without any further prompts and input from you. When installation has finished, you will be prompted for more information. The subsequent screens are self-explanatory, so just follow the instructions provided.