Before installing Altium on your PC you will need to have been added to the Altium Users and Groups and have a licence allocated to you, an email will have been sent from Altium if this has been done requesting for you to fill in your account details and provide a password which you must complete. Contact your account manager if you have not yet been given licence details or if you have not yet had an Altium account email.

Go to Altium.com->Resources&Support->Downloads were Altium Designer should be top of the list, click on the "online" installer link which will download the latest installer, you will be asked for your account login at some point.

Run the Altium installer file and install Altium with the default settings on your to C drive. Note: if you are replacing an older year revision of Altium it is best to install in the suggested new version directory, you will also be asked if you wish to import the settings from your older installations which is recommended. Close Altium once it has installed and opened.