If you cut accurately, then the most likely culprit is the 1/4" seam. It is essential to find where the 1/4" is on your machine.
http://www.quiltingboard.com/t-109768-1.htm is a tutorial.
Pressing without stretching is another fun one to learn, but important.
http://www.quiltingboard.com/t-149984-1.htm is a tutorial.
Learning how to "nest" the seams really helps with getting blocks and rows to line up.
http://www.quiltingboard.com/t-28328-1.htm is a tutorial.
Looking through the tutorial section of QB is a great way to find out how to do just about anything!!!