You can use Remote for iOS to control iTunes on your Mac or PC via iPhone/iPod/iPad. Remote allows you to play audio and video on a remote computer which is running iTunes.
First, install Remote app on your iPhone/iPod/iPad. Then, open iTunes on your computer, and turn on home sharing. This will allow Remote to connect to iTunes library on that computer.
To turn on home sharing in iTunes 11, go to File -> Home Sharing -> Turn on Home Sharing. If you are using older version of iTunes, go to Advanced -> Turn on Home Sharing.
In Remote app, go to Settings, and turn on Home Sharing. Enter your Apple ID, and Remote will detect any computers running iTunes under that same Apple ID that have Home Sharing turned on.
You can also add iTunes library to Remote app without having to turn on Home Sharing on your Mac or PC. In Remote app, go to Settings and select Add an iTunes Library. This will give you a Passcode, which you will then enter in iTunes. Open iTunes and select your device that is running the Remote app from Devices list, and enter the Passcode.
Remote can also be used to control AppleTV devices (1st, 2nd and 3rd generation devices). You can find more information about setting up AppleTV and Remote here.