The first thing you should do before joining Foursquare is check to see if they cover your city. Upon its initial release, Foursquare had coverage for just 12 cities. The founders continue to add more cities to the service; however, if your city isn't on the list, the application won't be very useful.
Assuming your city is covered, all you have to do to create an account is visit the Foursquare Web site. There you'll fill out a simple online form to get your free account. You can also upload a photo to act as your account's avatar. Now you're ready to use Foursquare!
The way you use Foursquare depends on the type of phone you use. If you have an iPhone or Android-based phone, you can download an application that takes care of everything for you. Foursquare founders say they hope to have applications for other smartphone operating systems like Palm's WebOS or the BlackBerry OS in the near future. If your smartphone isn't supported but has Web-surfing capabilities, you can visit Foursquare's Web site. If you're stuck with a regular cell phone, you can still use Foursquare. Just send a text message to Foursquare's SMS code of 50500.
Let's say you have an Android smartphone with a Foursquare app installed. How do you use it? You need to hit the town and visit a location. That location could be a bar, restaurant, club or just about anything else. You'd activate your Foursquare app, which taps into your Android phone's GPS receiver to get an idea about where you are. The app will pull up a list of locations near you. You choose the one that you'll be visiting and the app updates your status.
Update your status at a location enough times and you become the mayor of that spot. Visit enough locations -- or enough of the same type of location -- and you may earn a badge. If your friends also have Foursquare they can see your updates and keep track of what's going on. You might even meet up with them later in the night as you converge on a particular location.