You can choose any yarn you like the look of for the baby's afghan. It is always a good idea to choose a kind of yarn that is soft rather than scratchy, especially when you are making something to be used by a small child. Good luck with your project.
Softer yarns work best for baby afghans. Most craft stores or places that carry yarn have special yarn that is for baby projects. They come in a variety of softer colors that look good and are soft. They work well for afghans, sweaters, hat and any other baby apparel or garment.
Use worsted weight yarns. Your starting chain should be a multiple of 11 plus 6 stitches. you can easily adapt this pattern to any size you like. You can start practicing with afghan squares and do multiple designs of your wish!! it's quite easy.
For a baby afghan, it is best to use a yarn that is machine washable. Try cotton or acrylic blends; most companies sell soft, lightweight yarns marketed for babies.