what kind of application is it? is it one that you drag to your Applications folder to install or one with an installer? if the former just drag it to the Trash otherwise you should be able to uninstall it inside the installer or there may be a separate uninstaller. if not check the developer's site for uninstall instructions.
applications that you drag to the Trash to uninstall may have small preference files, application support folders and caches left over that are usually negligible in size. if you want to remove them search for the app name in a Finder search with "System files - included" added and Trash the appropriate files. you can also use an applications such as AppCleaner to do this.
Short answer: Drag the program(s) you want to get rid of to Trash. (Unless it was installed using a dedicated installer; in witch case it should also have an uninstaller.)