To draw the structural formula of hexene, start with six carbon atoms in a line. Draw a double bond between the first and second carbon atoms, and then add single bonds between the rest. Then, add hydrogens to all the carbons, making sure that each carbon shares four bonds total.
Hexene can also be drawn by having the double bond between the second and third carbons instead of between the first and second. This isomer is called 2-hexene, while the isomer described above is called 1-hexene. While hexene by itself usually refers to 1-hexene, there are other ways of representing the molecule.