Well, its an evolution thing. Basically the world started out as a giant rock with lava pools all over it. After a while the particles within the atmosphere combined to form water (H2O), after a long while this H2O became very abundent on the earth and basically enclosed a single land mass with water. After a while this land mass split into many land masses, with 6 main "blobs" (for lack of a better word). These 6 blobs became the continents as we know them today. The slowlyy spread apart to form the oceans we see today. I hope this helps, if not any age 15ish geography book should tell you a more detailed picture.

It seems that there isn't a universal consensus on their origin. There are diverse theories. "As Earth formed in a cloud of gas and dust more than 4.5 billion years ago, a huge amount of lighter elements, including hydrogen (H) and oxygen (O), became trapped inside the planet as the gases condensed and formed molten rock. Materials of different densities separated out; in the young planet's molten interior, heavy elements sank and light elements rose. Gases rose through thousands of miles of molten and melting rock, to erupt on the surface through volcanoes and fissures. Within the planet and above the surface, oxygen combined with hydrogen to form water (H20). Enormous quantities of water—enough to fill oceans if it were liquid—shrouded the globe as an incredibly dense atmosphere of water vapor. Near the top of the atmosphere, where heat could be lost to outer space, water vapor condensed to liquid and fell back into the water vapor layer below, cooling the layer. This atmospheric cooling process continued until the first raindrops fell to Earth's surface and flashed into steam." Source: http://science.jrank.org/pages/4824/Ocean-Origin-ocean-water.html Also: "A “standard model” describes the formation of the Earth and inner planets from the agglomeration of planetesimal masses (Wetherill, 1990; Taylor, 1997). However, the time period from the formation of the Earth (4.5 Ga) to the establishment of plate tectonics at the beginning of the Proterozoic (2.5 Ga) remains largely hidden and unknown. During this era, the ocean, atmosphere, and continents formed, and the geochemical and physical processes that allowed the development and evolution of life were established. Our knowledge of these beginnings is scant, with theories changing as each new line of evidence emerges." Source:http://docserver.ingentaconnect.com/deliver/connect/bell/00206814/v44n2/s3.pdf?expires=1187479961&id=39071597&titleid=6470&accname=Guest+User&checksum=C0E0CE3C1312811B22A88B580B1F3A87

The problem is rather the other way round: how did dry land come to be. If you imagine a perfectly spherical dry Earth, then the first bucket of water will spread in an even film over the whole surface, and even relatively small amounts will cover the whole Earth. Given the massive amounts of water, it is more suprising that anything sticks out above water level.