Use the Tabs pane of Safari preferences to set options for opening webpages in tabs.
Open pages in tabs instead of windows
Some links on webpages open in new Safari windows instead of replacing the current webpage. Choose Never to have such links open in new Safari windows instead of tabs. Choose Always to open new tabs instead of windows.
Choose Automatically to have Safari open pages in tabs unless they’re designed to fit in specially formatted separate windows; Safari opens those pages in windows instead.
⌘-click opens a link in a new tab (Mac)
Ctrl-click opens a link in a new tab (Windows)
To change the shortcut for opening a link in a new tab, select or deselect this checkbox.
When a new tab or window opens, make it active
To have new tabs or windows open in front of the one you’re currently viewing, select this checkbox.
Some keyboard shortcuts change depending on which options you select in Tabs preferences. The list at the bottom of the Tabs pane shows you the keyboard shortcuts.