How to center a clip on Premiere
To centre the video clip:
Select the clip in your timeline
Click on the Effect Controls tab in the left panel
Open the Motion section by clicking on the triangle, and adjust the position of the clip.
To make it central relative to your sequence, it will need to be half the values of the sequence.
So if your sequence settings are 1920x1080, you would need to choose a position of 960x540
To align the image on top, you can use the same method.