Go to this site:
http://www.microsoft.com/windows/windows-7/download.aspxYou will need a MSN (Microsoft Network) account. If you don't have one create it. It's free.
After you sign in, you will be taken to a page with a "Product Key" you can use. Copy it and write it down, print it, keep it somewhere safe. You will need it during the install
Start the download. It will take an hour or more... Their servers are VERY busy and it's a big file
Remember, If you are running XP or Vista, an "upgrade" in place is possible. (but I HIGHLY recommend against it) If you are running an earlier version of Windows 7, you MUST do a clean install or you WILL have problems.
Enjoy... Windows 7 is just awesome and I highly recommend it.