Double bonds will alter the geometry around an atom by basically decreasing the number of bonds that atom will form.
For example, in methane (
here the central carbon atom forms bonds with four other atoms. The bonding pairs of electrons will spread out in three dimensional space to get as far apart from each other as they possible can (VESPR theory). This molecule will have a tetrahedral geometry (
In a molecule where carbon forms a double bond like formaldehyde (