Any type of yarn will work for making practice swatches, but some types of yarn are easier to crochet with than others. You can use any kind of cotton or otheraran fiber weight yarn to make your swatches, and many people also like to use wool yarn when they're first starting out because it is so forgiving. If you're new to crochet, however, you might want to start with a less-complicated stitch such as the basic crochet stitch before moving on to more complex stitches.