Rulers are useful for checking the position of elements in your composition. Choose View>Show Rulers or Press Cmd+R (Ctrl+R) to bring them out. Once they're out, rulers can unlock a lot of useful features.
You can adjust the rulers' center point by clicking in the upper left corner of the comp window. Click and drag to reposition the rulers.
You can drag guides from the rulers for use in your composition.
You can turn Snapping on for guides, by pressing Cmd+Shift+; [semicolon] (Ctrl+Shift+;).
To toggle guide visibility, press Cmd+; [semicolon] (Ctrl+;).
To lock guides in place, press Cmd+Option+Shift+; [semicolon] (Ctrl+Alt+Shift+;).