If you have minor children you need to have a Will to name a guardian who will care for your children in case both parents are killed or incapacitated. If you have property to leave to minor children or have property that you care about (including things of sentimental value), you need to have a Will or some other form of estate planning. If you want to give instructions for medical care if you should become incapacitated or to choose who should make those decisions for you, you need a Living Will, which isn’t really a Will at all, or a Durable Power of Attorney (also called a Power of Attorney for Health Care).
A Will needs to be prepared and properly executed (signed by the principal and witnesses) while you still have legal capacity. This means you must decide on the terms and actually sign the Will before age, disease or accident make you unable to make those kinds of decisions. Legal capacity is defined by the laws of your state.
If you want to name a guardian for your children, make sure your brother gets your car or that your daughter gets your diamond necklace, then the sooner you plan your estate the better. This almost always involves executing a Will of some sort. If you wait until an accident or an illness strikes, it could be too late.