To be comfortable shooting manual you will need to know what your necessary adjustments are. Though these settings are all numeric, there isn't some insane math equation that you need to do to find out what to dial
The COOPH put together a great video that illustrates 9 useful composition techniques as seen in legendary photojournalist Steve McCurry’s photography. * You can get additional details at the image link.
Shutter Speed aka (exposure time) , stands for the length of time a camera shutter is open to expose light into the camera sensor - is one of the three pillars of photography, the other two being ISO and Aperture