The sum of the external angles of a polygon is always 360°
To find the measure of one angle, the formula is 360/n where n is the number of the sides of the polygon.
360/6 = 60°
The sum of all internal angles of a polygon can be find with the formula (n-2)*180° where n is the number of the sides. In this case 180(6-2) = 180*4 = 720°
To find one internal angle, follow this formula [180(n-2)] / n = [720/6] = 120°
Anyway, you can pick a vertex and from that vertex start to draw diagonals.
There will be some triangles, the sum of all internal angles of a triangle is 180°. In a polygon, the sum of all internal angles equals to the sum of all internal angles of all the triangles.
Hope I'm clear.