What is the overall polarity of the molecule Formaldehyde; why?

Formaldehyde is an aldehyde which means that the carbonyl group is the predominant group

--  O
-   II  
H-C-H

Formaldehyde has a highly electronegative oxygen and a carbonyl carbon.

There is a significant electronegative difference between Oxygen and Carbon. In which electrons are pulled more strongly towards the Oxygen end (electrons in the double bond)

This creates a partial negative charge on the oxygen (because electrons in double bond are closer to oxygen)

And since there is less electron influence around the carbon (the carbon has a partial positive charge)

This gives them positive and negative poles and thus it is a polar molecule.