Sometimes the feed dogs are aligned so they pull the fabric at a slight angle.
A friend taped a full post-it note pad (with the glued side toward the needle) at a 1/4" distance and properly aligned on her machine bed. She uses it as a guide for her fabric. BTW, she taped the pad down with wide blue painter's tape.
If you are a real beginner, it may also be a matter of just getting familiar with all the bits. As long as you do not have HSTs in your design - only straight cuts, a consistent seam allowance will work for you. As soon as you introduce HSTs, you need that 1/4" or you'll cut off the points.