Add the following to your .emacs file:
(require 'highlight-symbol)
(global-set-key [(control f3)] 'highlight-symbol)
(global-set-key [f3] 'highlight-symbol-next)
(global-set-key [(shift f3)] 'highlight-symbol-prev)
(global-set-key [(meta f3)] 'highlight-symbol-query-replace)

Use `highlight-symbol' to toggle highlighting of the symbol at
point throughout the current buffer.  Use `highlight-symbol-mode' to keep the
symbol at point highlighted.

The functions `highlight-symbol-next', `highlight-symbol-prev',
`highlight-symbol-next-in-defun' and `highlight-symbol-prev-in-defun' allow
for cycling through the locations of any symbol at point.  Use
`highlight-symbol-nav-mode' to enable key bindings (M-p and M-p) for
navigation. When `highlight-symbol-on-navigation-p' is set, highlighting is
triggered regardless of `highlight-symbol-idle-delay'.

`highlight-symbol-query-replace' can be used to replace the symbol.