Check each atom to see if it obeys the Octet Rule which says that an atom in order to be the most stable must have eight valence electrons with the exception of Hydrogen which must have only two electrons in their valence region to be stable. If any valence electrons have yet to be assigned or atoms do not aparently follow the Octet Rule then check for possible multiple bonding where more than one bonding pair may exist between two of the atoms in the structure. It is possible to convert a lone pair into a bonding pair in order to assure that an atom will follow the Octet Rule. There are exceptions to the Octet Rule such as Boron and Aluminum atoms needing only six electrons in the structure. Sulfur does this quite often and so does Xenon one of only two Noble Gases that chemically combine to form compounds.
Lets try another.
Determine the total valence electrons for all atoms indicated in the given molecular formula. If there is a negative charge add one electron to the total for every negative charge. If there is a positive charge indicated on the molecular formula subtract one electron for every positive charge indicated. In this compound there would be 3 carbons for 12 and 6 Hydrogens for 6 more or a total of 18 valence electrons total. Determine which atom or atoms will be in the center.