If you are searching for a way to download and install the Insider Preview (the not finished product) then follow this:
- First, you will need a Microsoft account. If you have a Hotmail or Outlook e-mail account, then you already have a Microsoft account.
- Go to the Windows Insider web page and click on the "Get started" button.
- Follow the steps on the web page until you reach a page where you can choose to download Windows 10 Preview for PC.
- Download it and install it.
In my opinion the web page is very self-explanatory and easy to use.
If you are looking for the free upgrade to Windows 10 (the finished product) then you should look for a small Windows logo in your Notification area (a.k.a. the System tray). If you click that icon you can reserve a copy of Windows 10. But you won't get it until the end of July.
Hopefully this helped you with finding what you wanted.