You have two options for this:
1) Change the camera resolution in your camera settings. You have the option to do 9.1M or 4.7M in 1:1 ratio.
2) Take the picture as you usually do with your preferred camera resolution and then view the picture in your Gallery (or the preview right after taking it), tap the crop/rotation icon (first icon on the bottom left) and select the square crop. Adjust the position and done!