A lot of apps you can just drop the app itself in the trash, but some leave behind junk. Myself and many others on here use the free app AppCleaner and it does a good job of cleaning everything up when you remove an app.
3. Run a cleaning program as weasel suggests to remove all those 'left overs'. AppCleaner v. 3.4 is great and shows what ityis eliminating by way of associated and left over files.