To install Windows 7, you'll require either the Windows 7 installation disc or the purchased online installation file. Additionally, have the 25-character Windows product key handy, found on the disc holder or in the confirmation email from the online purchase.
If installing on a Netbook or PC lacking a DVD drive, an external DVD drive will be necessary.
Prepare for the Windows 7 installation. Given the various installation scenarios, detailed instructions are available in Microsoft's tutorials.
For upgrading from Windows Vista, follow Microsoft's Upgrade from Windows Vista tutorial.
For a custom (clean) installation from Windows XP, refer to Microsoft's Upgrade from Windows XP tutorial.
Consider seeking assistance from a computer service professional for a custom (clean) installation if needed