You need a will. It will properly explain your wishes to everyone you leave behind, sparing your family emotional strain during an already difficult time. If you have children, a will is an absolute must. If you and your spouse were both to pass away, this is the document that will say who you want to become their legal guardians. That’s definitely not something you want left up to the courts.
A lawyer can help you draw up your will, or you might consider using a will-making computer program. Once you have the will, take a look at it every few years and update it whenever there’s a significant change in your life, like marriage or divorce, a new child, or a change in your preferences.