How change CharacterAlignment of Text in a TextFrame ?
It sounds a bit like you want to know about setting the text inset of a text frame. If so, select the text frame and go to Object>Text Frame Options (Command B is the shortcut) and go to the Inset Spacing fields. If you click the lock icon, all of the fields will change together with one value, but if you want different values for the four sides, uncheck the lock icon.
The fields require a number, so if you need an Em space, you will have to calculate the value based on your font size.