In many cases you will be added to a site by that site's administrator and you will receive and email informing you of your new site membership. Simply log in after receiving this email and your new membership will appear on your dashboard and on your Sites page.
If the site you wish to join is open or restricted (password protected) then you are able to join the site by searching for it on the Sites page.
If your site is labeled as Open then you are free to join the site straight away! Note that you may not be able to access resources if your account requires addition to specific user groups prior to access.
If your site is labeled as Restricted then you are required to enter a password, also known as a Join Code, in order to access the site. This join code would have to be provided to you by a site administrator.