The earth formed around 4.5 billion years ago. What we think happened was that gas and rocks floating in space collided and came together to form a mass that eventually got big enough and became the earth. It wasn't until another few billion years later that the earth started to look like it does today, with oceans and continents.

I've included a video clip from the national geographic which shows what we think the earth would look like when it first formed.

I'm not sure we know all of the details, but Earth formed from matter that condensed into a spinning disk the center of which made our sun. Whether the Earth was formed by a collapsing cloud of dust particles or was built up by the impacts of a great many asteroids I don't think we know.