The name 'Genesis' comes from the Latin word for 'beginnings'. And Genesis is indeed a book of beginnings - the beginnings of heaven and of earth, of man, and of sin and redemption from sin. It is the story of the birth of nations, and of the beginnings of God's dealings with people, primarily through the agency of Abraham. And it is the story of the birth and development of the nation of Israel. In Genesis, we receive our initiation into the framework underlying all that Scripture will subsequently reveal concerning God's activities.
Scripture begins with a number of presuppositions.
A presupposition is simply something which is pre-supposed, that is something which is assumed to be true without the bother of having to prove it. Philosophers refer to this as 'a priori' truth - that which is known before hand. Actually, in this verse we have 3 presuppositions. First and most basically, the Bible assumes 'a priori' the existence, the eternality and the knowability of God. God created all things at the word of his command, and he did so 'ex nihilo' - out of nothing.