Receiving an email with your serial number which begins with P3- and a download link - check your spam folder if you do not receive one.
Following the download link in the email or download the trial version and run the installer(s) - this is a 2 part install, the first being the Download Manager, the second the main program.
Start and activate the program with the serial number you received - the computer must be connected to the Internet.
Download and installed the extra content that becomes available under Help, Install extra content and the manual - also available under Help.
If you are new to Movie Studio then see the tutorials listed here - they refer to Movie Edit Pro which was the former name of Movie Studio.