To setup a Foursquare account go to www.foursquare.com.
You will now be given the option to download the Foursquare App for you model of phone. They of course cover the top 3 iPhone, Android and BlackBerry. But they have the app available for many different handsets.
The next step it will go over some of the options you have with your account. You can link your account to your Facebook and Twitter accounts so you have the option of sharing your location with your friends and you can also post comment this way.
Your account is now setup. You will use Foursquare nearly 100% of the time on your phone. So get out there and start checking in, Leave Tips for your friends and getting some great special offers.