You can start by paying attention to your breath. Your breath is a good starting place because it is with you always and it forces you into the moment.
As AuspiciousEight suggested, don't try to stop your thoughts. This is futile and will prove extremely difficult and frustrating. Instead, direct your thinking to watch your breath.
Also, if you start analyzing your breath, instruct your mind to pay attention to the air as it passes through your nostrils only.
When you are able to do this consistently, you can try a few other techniques.
As a caveat, there are many means and reasons for "meditation". Most of the western world considers meditation the slowing of the mind for stress relief purposes. This is a good start. However, if it's clarity you seek, bringing your attention only to the present moment is the beginning of the level of meditation you want to practice.
Check out
Awareness Meditation, if you want something guided.