Before cutting most fabrics, it's important to consider shrinkage. After splurging on a designer cotton print, I once made a tablecloth without preshrinking the fabric. After washing, my tablecloth was too small for my table. That tablecloth eventually became two throw pillows and a pile of expensive scraps.
Always know the type of fabric you are buying. Find the care information on the end of the fabric bolt, or on a hang tag if the fabric is on a dowel. Make note of it. As a beginner, it's easy to be unsure of your fabric. Rayon and cotton, for example, can look similar but require totally different laundering methods and iron temperatures.
Launder or clean your fabric as you would the finished product. If you plan to wash the finished item in the washing machine and dry in the dryer, preshrink it in the same way. Take care with dark fabrics where colors can run or bleed; don't mix them with lighter fabrics until you know for sure. If your fabric is 100% cotton, you may need to wash it twice. Cotton does not always shrink completely in one wash/dry cycle.
Even dry-clean-only fabric may need to be preshrunk.
Don't Forget to Preshrink Linings
If your project requires a lining or backing fabric, don't forget to preshrink it too. Otherwise, you end up with a project that won't hang or lay properly after laundering.
Interfacing & Notions
It's necessary to preshrink interfacing to avoid the distortion and puckering that can occur when the finished project is laundered. Preshrinking fusible interfacing will eliminate the ripples that can show up during the fusing process. Non-fusible interfacing can be laundered as your fabric and lining. Preshrink fusible interfacing by submerging it in warm to hot water until totally saturated. Blot between towels (never wring) to get out some of the water. Lay flat to air dry (hanging can cause distortion).
Notions like zippers, trims and elastic should be preshrunk following the manufacturer's instructions. If they are washable, wash as you did your fabric. A lingerie bag helps contain small pieces.
Press Your Fabric
Before you cut, remove fabric wrinkles by pressing with your steam iron. Cutting wrinkled fabric can cause distorted pieces that don't match up properly, making your finished project less professional looking.