Uninstalling a Mac application seems like an easy task -- just drag the icon from the Applications folder to the trash. This doesn't get rid of all of the files associated with that application, however. It might not always be necessary to get rid of all of an app's remnants, but if you're trying to reinstall an app that isn't working properly, you may need to take some extra steps first.
First, take a look in the Mac's Application Support folder. You can find this by holding down the "option" key, and clicking on "Go" from the Finder menu in the top menu bar. Click on "Library," and you should see the Application Support folder there. Find the application that you want uninstalled, and drag that folder to the trash.
Another option is downloading a small application called App Cleaner from freemacsoft.net. This app will find all of the files associated with a particular application, and it will clean them off for you.