Why does this happen... usually because they did not cut the fabric squarely to begin the project or they did not measure accurately as they assembled the item. Basic sewing tools are what you need to get started but other tools assure things like a square when you are measuring and cutting out fabric.
Using rotary cutting tools properly can assure that you are cutting squarely but when you are just learning to sew, cost can stop you from having every possible sewing tool... so let's explore the options you have around the house that can assure you are cutting squarely.
Your first step will be to use paper to make a pattern rather than immediately cutting your fabric. You can invest in pattern grid fabric which allows you to use the markings to make your pattern pieces squared off. But... the most economical way is to use things you have around the house. A few examples of things you have around the house that will make good pattern making material are, brown bags, newspaper and gift wrapping tissue paper. Gift wrapping can also be used but be careful about the colors from the wrapping paper bleeding or rubbing off on to your fabric. You want a pattern making "material" that will lay smoothly without bunching up, so plastic grocery bags are not a good choice.
As you draw the pattern it can "look like" you have a square corner but unless you are using a tool, you can be drawing a parallelogram instead. If you have someone in the household that does woodworking, there may be a carpenters-square available to borrow. Use the carpenters square to make sure your corners are truly square.
If all else fails look around. Your printer paper, pads of paper and almost all purchased paper or poster-board is manufactured with square corners. Boxes are usually square. Once you find a tool to use for square corners, make yourself a template that you can keep with your sewing tools. A durable piece of plastic or cardboard that will maintain its original shape is a good choice to make your template.
One way to test if your pattern is truly square, is to fold it. Matching the opposite edges, fold it in half and then in half again from the opposite directions. If all of your edges line up with center folds folding the pattern, you have a true square. If they do not or tucks in the paper are needed to make the edges lined up, you don't have a square and you need to re-work your pattern.
Accurate and consistent measuring are an important part of ending up with a usable pattern or even results when you are sewing. Use the same tool through out constructing a project. Measurement marking can vary on different tools and using the same tool, assures that you are repeating the same measurement.
The straight edge is going to be on the straight grain of the fabric. A line can be drawn parallel to the edge to give you a grain-line to follow.
Above all, take your time and double check before you cut out your fabric. There are many ways to accumulate the tools that make sewing easier, without breaking the bank. Learn about the ways to save money on sewing tools and by sewing.
