when did its formation start?
Ga ago
Earth
The oldest material found in the Solar System is dated to Ga (billion years) ago. By the primordial Earth had formed. 
what is earth? Third planet from the Sun what is its shape? Nearly spherical
wiki:1595152
Earth [SEP] Formation
The oldest material found in the Solar System is dated to Ga (billion years) ago. By the primordial Earth had formed. The bodies in the Solar System formed and evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, and dust (including primordial nuclides). According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10– (Mys) to form. A subject of research is the formation of the Moon, some 4.53 Ga.