According to About.com, the Lewis dot structure for H2CO, or formaldehyde, is drawn by placing a C (carbon) in the middle, putting one H (hydrogen) on either side and putting an O (oxygen) on top. This configuration contains 12 valence electrons arranged around the four atoms.
The link between the C atom and each respective H atom contains two electrons, represented by two dots or one straight line. The link between the C atom and the O atom contains four electrons, represented by four dots or two straight lines. The remaining four electrons are placed on either side of the O atom.