On the Tools menu, click Angle Snap. The default snap angle is 15 degrees but this can be changed on the Grid dialog. The snap angle must be in the range 5 to 90 degrees. When grid snap and angle snap are both enabled, grid snap takes priority.
When you draw a wall, the software will align it to multiples of the snap angle, and in most cases you are advised to keep angle snap active. Snap angles will be calculated based on the grid angle.