Because there aren't many atoms in space, there's not much to stop things from moving. For this reason, the Earth keeps on spinning and doesn't get much slower. (In truth, it's actually slowing a little because it's exchanging energy with the moon, which is why we have tides.) For the same reason, gravity from the sun is attracting the Earth towards the sun, and the Earth is moving quickly, so it causes it to orbit. Because there is nothing to apply an external force on the Earth and change its momentum, it continues in its stable orbit.