Settings .. Apps .. select the app and tap the Uninstall button.
Note that some apps cannot be uninstalled .. easily. This includes some Google apps, manufacturer apps and - if your phone is network subsidised - some network apps. To uninstall these (not always recommended with Google and manufacturer apps) you will need to root your phone. Go to the Galaxy Ace sub-forum and then the rooting section for more information.
If you just mean remove the app from your screen rather than remove it entirely, press and hold until the screen changes then drag the icon to the top of the screen and drop it on the x Remove bar
