how to enable vertical sync in opengl?

On Windows there is OpenGL extension method wglSwapIntervalEXT.
From the post by b2b3 http://www.gamedev.net/community/forums/topic.asp?topic_id=360862:

If you are working on Windows you have to use extensions to use
wglSwapIntervalExt function. It is
defined in wglext.h. You will also want to
download glext.h file.
In wglext file all entry points for Windows specific extensions are
declared. All such functions start
with prefix wgl.
To get more info about all published extensions you can look into
OpenGL Extension Registry.

wglSwapIntervalEXT is from WGL_EXT_swap_control extension. It
lets you specify minimum number of
frames before each buffer swap.
Usually it is used for vertical
synchronization (if you set swap
interval to 1). More info about whole
extension can be found here.
Before using this function you need query whether you card has
support for WGL_EXT_swap_control and
then obtain pointer to the function
using wglGetProcAddress function.

To test for support of given extension you can use function like this:

#include <windows.h>
#include "wglext.h"

bool WGLExtensionSupported(const char *extension_name)
{
    // this is pointer to function which returns pointer to string with list of all wgl extensions
    PFNWGLGETEXTENSIONSSTRINGEXTPROC _wglGetExtensionsStringEXT = NULL;

    // determine pointer to wglGetExtensionsStringEXT function
    _wglGetExtensionsStringEXT = (PFNWGLGETEXTENSIONSSTRINGEXTPROC) wglGetProcAddress("wglGetExtensionsStringEXT");

    if (strstr(_wglGetExtensionsStringEXT(), extension_name) == NULL)
    {
        // string was not found
        return false;
    }

    // extension is supported
    return true;
}

To initialize your function pointers you need to:

PFNWGLSWAPINTERVALEXTPROC       wglSwapIntervalEXT = NULL;
PFNWGLGETSWAPINTERVALEXTPROC    wglGetSwapIntervalEXT = NULL;

if (WGLExtensionSupported("WGL_EXT_swap_control"))
{
    // Extension is supported, init pointers.
    wglSwapIntervalEXT = (PFNWGLSWAPINTERVALEXTPROC) wglGetProcAddress("wglSwapIntervalEXT");

    // this is another function from WGL_EXT_swap_control extension
    wglGetSwapIntervalEXT = (PFNWGLGETSWAPINTERVALEXTPROC) wglGetProcAddress("wglGetSwapIntervalEXT");
}

Then you can use these pointers as any other pointer to function. To enable vync you can call wglSwapIntervalEXT(1), to disable it you call wglSwapIntervalEXT(0).

To get current swap interval you need to call wglGetSwapIntervalEXT().