First, you should press Start -> Windows updates and select 'Check for updates'(this should take a while) then 'Install updates' (restart may be required). You may need to 'Check for updates' multiple times until 'Windows is up to date'.
Second, insert your Windows 8 Installation disc in your DVD or BD reading device, wait for the AutoPlay window to pop-up, click 'Run setup.exe'. Before the installation begins, you will be asked for a product Key that is on disc package, if not, contact your disc provider. Click next, read and accept the 'License Terms', click next, and select 'Keep windows settings, personal files and apps' then click next and install.
Wait for the process to complete and you should have Windows 8 installed.