To start upgrading, visit the following link and download the appropriate download tool:
Once it has downloaded, double-click on it to execute the program.
At the opening screen, you can choose to automatically upgrade to Windows 10 or create a bootable ISO or USB flash drive to upgrade another PC. Upgrading is a much easier process as it will automatically detect your product key and install Windows 10 over your existing Windows installation. For those, though, who wish to perform a clean install, you will need to find your product key first. Information on how to do this can be found in Microsoft's Windows 10 FAQ.
If you have any questions regarding this process, feel free to ask for help in this topic or the Windows 10 forum.