Just to add to Terry response.
All layers etc… in the timeline have transform properties. The position values in there can be inputted directly and AE will perform basic maths for you too.
For example, if you want a layer exactly in the middle of a 1920×1080 comp, you can click on the X value and type:
1920/2
And AE will set the X to 960
And you can do the same for the Y value, 1080/2.