Just to make sure I understand: You are making a 9-patch and it is not really square. Hm, I would measure each section to see what part is off.
It is possible that you cut one of the sections not to size. It is possible that your seam allowance is off.
When you have the final size of your unfinished 9-patch, cut the alternating squares to the same size and you will be fine.
I usually make sure that all of my pieced blocks are squared up to the same size.
Most importantly, you are a true beginner. Don't give up and don't be upset. Maybe it is time for you to get acquainted with the frog tool also known as the seam ripper (rip-it, rip-it) WE ALL know it well.