You can view multiple webpages in a single Safari window by opening them in tabs.
Open a new tab: Click the New Tab button 
near the top-right corner. To change the page that opens in new tabs, choose Safari > Preferences, click General, click the “New tabs open with” pop-up menu, then choose an option.
Open a page in a new tab: Command-click a link on a page, a Favorites icon, or a Top Sites thumbnail. You can also Command-click the Back button 
or Forward button 
to open the previous or next page in a new tab.
You can also type in the Smart Search field, then Command-click an item in the search suggestions, or press Command-Return.
Tip: To use these Command-key shortcuts to open pages in new windows instead of new tabs, choose Safari > Preferences, click Tabs, then deselect “⌘-click opens a link in a new tab.”
Open in new tabs from the bookmarks sidebar: Control-click a bookmark, then choose Open in New Tab from the shortcut menu. Or, Control-click a bookmarks folder, then choose Open in New Tabs.
To open the bookmarks sidebar, click the Sidebar button 
in the toolbar, then click the Bookmarks button 
.
Pin a website: Drag the tab with the website you want to pin to the left side of the tab bar. When the tab shrinks and displays the website’s icon or initial, drop it in place.
Show all tabs in a window: To see thumbnails of all your open tabs, click the Show All Tabs button 
, or pinch closed on a trackpad. To view one of the pages full size, click its thumbnail. To close a tab, click the Close icon 
in the upper-left corner of the thumbnail.
Switch tabs: Click a tab in the tab bar. When there are more tabs than fit in the tab bar, swipe left or right over the tab bar to scroll through your tabs.
You can also press Command-1 to Command-9 to select one of your first nine tabs. Turn on the preference in the Tabs pane of Safari preferences.
Open a tab in another window: Drag the tab over the desktop or choose Window > Move Tab to New Window.
You can also drag the tab of one Safari window to the tab bar of another.
Note: Tabs in Private Browsing windows can only be moved to other Private Browsing windows. Non-Private Browsing tabs can only be moved to non-Private Browsing windows.
Convert all Safari windows to tabs in a single window: Choose Window > Merge All Windows.
Automatically open pages in tabs or windows: Choose Safari > Preferences, click Tabs, click the “Open pages in tabs instead of windows” pop-up menu and choose an option:
Never: Links designed to open new windows will open new windows.
Automatically: Links designed to open in new windows will open them in new tabs.
Always: All links designed to open new windows will open in new tabs, including links designed to open in specially formatted separate windows.
Close a tab: Place the pointer over the tab in the tab bar, then click the Close icon on the left side of the tab.