There are various types of yarn. Yarns are categorized based on weight from smallest to largest and types of fiber such as cotton, wool, and acrylic. When you choose yarn, try to not to choose very small or sophisticated yarn. The small and sophisticated yarns are attractive to knit but we are beginners. For beginners, larger simpler yarn is the easiest to knit. Then, you can select any colors that you like.
Based on different sizes of yarns, you can choose different size needles. Typically, the yarn suggests a needle size to use. See below photos.
There are straight needles and circular needles. Usually, when you knit the round, you need circular needles. Otherwise, you can choose the straight needle.