How to stop Safari opening every time the computer opens

Look in System Preferences > Users & Groups > Login Items and use the (-) button to remove Safari if it is there. Also check in Apple menu > Logout, Restart, or Shutdown and if "Re-open windows when logging back in" is checked then uncheck it.

In some cases this can happen if adware, anti-virus software or cleaner type application are installed.

To find out, run EtreCheck and post the report here.

      https://etrecheck.com

      Click  “Free download” button,

      Open Downloads folder, click on it to open, and then select ”Open”.

      “Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.

      Click “Share Report” button in the toolbar, select “Copy report” .

      Paste the report when you reply.