The set-up is the most important part of the golf swing because everything else reacts to it. 

If you were poorly aimed, for example, you wouldn’t have a good swing, because the ball would never go toward your target. Posture, spine angle, grip and alignment are almost equally important. To find a perfect posture followed by a perfect balance, try this type of exercise :
Keep your spine angle straight and hold the club parallel to the grass with your arms straight down. Bend slightly your knees and move forward with your upper body by bending your hips. Use a mirror to check your posture. If your posture is the red line is to highlight that your spine angle is not in the natural position . In the first example, the back is too bended at the top, causing an overuse of your upper body. In the second picture, the position of your back is called "hyperlordosis", which mean you are creating a "S" and the tendency is to force a pivot around your mid-spine without using the lower back as a fixed point. Both position will create most probably some injuries during your golf life . 

The correct knee flex can be determined by looking down on your knees and projecting a vertical line passing just in front of the kneecap, ending through the junction of the forefoot and midfoot. If the vertical line passes in front of the toes, then the knees are too bended, If the vertical line passes through the rear end of the midfoot, then the knees are insufficiently flexed. To keep the balance in the middle of your feet, your glutes/hips need to move back to compensate the upper body forward movement. It is important to bend your hips and not your waist. Note that if you bends your hips, the lower back will be straight creating a perfect position. 
As you can see below , the first posture is what every player as to achieve, respecting the correct angles and balance between upper body and lower body . The second and the third postures are the common errors of a poorly address position.

First Posture : 

The spine should have an angle (forward) of 35/40 degrees. In this case, you are respecting the normal position of your body with a perfect balance and you have more chance to have a neutral rotation, covering the ball with your right shoulder and transferring the power to the ball with no compensations. 

Second Posture : 

If the position is too upright, most of the time an over-the top move will occur . The balance and the spine angle is very poor and all of your power will not transfer into the ball. There is also less action and rotation of your hips due to the aggressive forward movement of your upper body . 

Third Picture : 

Posture that is to bend forward causes a severely inside-out swing path. Also the balance and spine angle are very poor and all of your power will go inside the ball, causing a curve right to left of the ball. 

Having said that, you have respect two important points: The arms should hang straight down from the shoulders, perpendicular to the grass and in front of your toes. If your arms are too far, the tendency will be to swing the club too early inside; if they are too close, the tendency it will be to move your club to early outside. The second point is regarding the grip, because the left hand is higher then your right hand, the left shoulder should be higher than the right shoulder. This also implies a little shift of your hips toward the target

The Body Alignment : 

Body alignment refers to the positioning of the body relative to the ball-target line. The body (feet, knees, thighs, hips, shoulders) should optimally be aligned parallel to the target line if you are looking for a straight shot. But the tendency of any player is to decide first which type of ball flight and trajectory you want and then align the body, for right handed players, a bit more to the left side of the target for a fade shot or more to the right side of the target for a draw shot (left handed players is the opposite). If the shoulders are pointing to the right of the target, then it is very likely that the arms will swing from in-to-out across the front of the body during the downswing. If the shoulders are pointing to the left of the target, then it is very likely that the arms will swing from out-to-in across the front of the body during the downswing .