For a baby afghan, you would probably use 3 yarns on size 15 needles, and the 50 stitch cast on. For a bed sized afghan, you'd probably use 5 yarns on size 19's and the 70 stitch cast on. Then begin pattern. K each stitch, wrapping yarn twice around needle to create an extra wrap. Pick one yarn of each color you would like to have in the afghan and use a variety of textures.
Cottons make a great beach throw.
Make a swatch before you begin to get an idea of how your yarns work together. It is surprising how the colors change when worked together. You may change you combination many times before you get the desired effect. That's it for now. Take care and happy knitting.