Drawing a lewis structure is the first step to determine bond angles.

For example, BCl3 is trigonal planar, and therefore the bond angle is 120 degree.  However, when a molecule is polar, then even when it is a trigonal planar shape, it can't have a bond angle of exactly 120 degree.

For example, CHClO. But in this case, the bond angle shouldn't deviate too far away from 120 degree because there's no lone pair on the central atom. Molecules that contain a lone pair on the central atom will cause repulsion and that is the reason.

If a molecule has two bonding groups and one lone pair electrons, the bond angle will obviously be less than 120 degree and cause a bend molecular shape. Lone pair electrons is the main reason why molecules that have the same electron-domain geometry can have different molecular geometries.