Step One: You'll need to create a new adjustment layer (Ctrl+Alt+Y).
Step Two: Make sure your new adjustment layer is selected, then use your pen tool (G) to mask out your desired shape.
Step Three: Search 'Transform' in the Effects & Presets tab, then apply it on to your adjustment layer.
Step Four: Select the bullseye icon next to the anchor point, then pinpoint to the middle of your masked shape.
Step Five: Copy the position properties with to the new anchor point properties.
(your properties will be different than the example)
Step Six: Now you need to go to the 'transform' effect on your adjustment layer and keyframe the scale to your desire look.
Tip: If you don't know what keyframing is, search a tutorial or video it's pretty simple.