Depends on what phone you have. To uninstall an app on an iPhone, iPad, or iPod, just press and hold the app until it jiggles and than hit the x in the upper left hand corner. To delete an app off of iTunes, click apps, click the app icon you wish to delete, and than click edit, and finally delete.
To delete an app off of android, go to your menu in the upper right hand corner, tap settings, go to applications, and finally manage applications. Select the app you wish to delete and hit uninstall.
For windows phone, you will want to go into settings, storage sense, click on either the phone or SD card (where the app is installed), click on apps+games, click the app you want to delete, and click uninstall. The following links go more in depth for hwo to do this.
Android - http://www.androidcentral.com/how-uninstall-apps
iOS - How to Delete an App from Your iPhone
Windows Phone - http://www.windowsphone.com/en-us/how-to/wp7/apps/delete-apps-and-games