Yarn weight and tool size have to match each other, and fiber choice affects how forgiving a project is to work with — two details that matter more to a beginner's actual experience than pattern choice or color.
Yarn weight must match hook or needle size
Yarn is categorized into standard weight categories, from very fine lace-weight yarn to super bulky, and each weight category has a corresponding recommended hook or needle size range printed on most yarn labels. Using a hook or needle size mismatched to a yarn's weight produces fabric that's either too loose and holey (needle or hook too large for the yarn) or too tight and difficult to work (too small for the yarn) — checking that yarn and tool size are matched, rather than picking each independently, avoids frustrating early results that have nothing to do with actual skill.
Fiber content affects both feel and how forgiving a project is
Acrylic yarn is inexpensive, widely available, machine washable, and holds up well to handling, making it a common recommendation for beginners specifically because mistakes are easier to work with and redo without damaging delicate fiber. Wool yarn has natural elasticity that makes stitches easier to see and work with for some beginners, and blocks (can be shaped with light steam or washing) well for finishing a project, but is typically more expensive and often requires more careful hand-washing. Cotton yarn has little to no stretch, which can make it noticeably harder to work with for a true beginner still developing consistent tension, despite being a popular, breathable choice for certain finished projects like dishcloths or summer garments.
Tension (gauge) affects whether a finished project matches its intended size
How tightly or loosely an individual knits or crochets — called tension or gauge — varies from person to person even using identical yarn and tool size, and a pattern's finished measurements assume a specific gauge that may not automatically match how any given person naturally works. For projects where finished size actually matters (a garment meant to fit, for instance), knitting or crocheting a small test swatch and comparing it to the pattern's stated gauge before starting the full project catches a mismatch early, rather than discovering an ill-fitting result only after finishing.
Knitting needle material affects how yarn behaves while working
Metal needles are slippery and fast, letting stitches slide easily — good for smooth, easy-gliding yarns but can let stitches slip off unintentionally for a beginner not yet holding consistent tension. Bamboo or wood needles have more grip and texture, holding stitches more securely and making them a common recommendation for beginners, particularly with slippery yarns that might otherwise slide off metal needles too easily.
The one thing people forget
Keep the yarn label from any project in progress, or note down the specific yarn weight, fiber, and dye lot used, since running out mid-project and needing to buy more of an unlabeled yarn can mean an inability to match dye lot or exact fiber blend later, resulting in a visible color or texture mismatch in the finished piece.