To get started, you’ll need the most recent version of the Facebook application for iPhone. You also can access Places from touch.facebook.com if your mobile browser supports HTML 5 and geolocation.
Go to Places on the iPhone application or touch.facebook.com site and then tap the “Check In” button. You’ll see a list of places near you. Choose the place that matches where you are. If it’s not on the list, search for it or add it. After checking in, your check-in will create a story in your friends’ News Feeds and show up in the Recent Activity section on the page for that place.