If you don't care about stored data within the app, the easiest thing to do is just re-download it with your iPhone. They won't charge you again as long as you use the same iTunes account.
If you DO have it in iTunes on your computer already, and would prefer to upload it from there, click on the iPhone in the lefthand side, and then select the Apps tab in the main window and make sure that Apps are set to sync with your iPhone.