A beginner's guide
Find a hashpoint
A simple method is to visit a Coordinate Calculator website and click on the map. There is much more detail here.
Visit the hashpoint
Go whenever you like but beware: Like Cinderella's coach, the hashpoint vanishes at midnight and a new point appears somewhere else. If you arrive at 4pm on Saturday, you are more likely to meet another geohasher. In most parts of the world, this is the official meetup time.
Re-visit the Coordinate Calculator, find the point you visited and click the Meetup link (bottom left). You will need to edit this page. If someone else has already created the page, you can add to it. More likely, it will be blank and you should copy the block below and paste it into your new page.
{{subst:Expedition |lat = 55 |lon = -17 |date = 2012-07-22 }}
Change the numbers to match your own latitude, longitude and date. Save the page. Go back into edit mode and fill in the blanks.