A good head of cauliflower will have tight florets of an even, pale white color. Yellow or brown spots should be avoided, though if they're minor, you can trim them off, no problem. Look at the leaves as well; they should remain tight around the base and appear bright pale green, with no yellow or wilted spots. As cauliflower ages, it'll start to get small black spots on it. These are not mold and are safe to eat, but they're a good indication that the cauliflower is past its prime.
Once you get the cauliflower home, keep it loosely wrapped in plastic or in a vegetable bag inside the crisper. It should stay good for at least a week. After you've cut it into florets, it's best to use it as fast as possible, though florets can also be stored in an airtight container for up to three days or so.Start by cutting the entire head of cauliflower into four quarters, cutting straight down the center through the core.