I know that it seems like a very easy question, but based on some of the exercises that I see people doing it seems worth considering. If you are using free weights (dumbbells, barbells, weight plates, etc.), the resistance force is the pull of gravity. For example, if you were going to perform shoulder rotation exercises with a dumbbell, then you would need to position yourself such that the movement would be against the pull of gravity. Standing with your arm at your side with your elbow bent at ninety degrees would not have the rotation against gravity. In this position, the resistance of the weight will not be working the rotator cuff muscles.
Similarly, with a starting position of holding weights in your hands with arms extended in front of you while your torso is vertical and pulling your hands toward your torso is not an effective way to work the muscles of your back.
Both of these examples I have recently witnessed and I feel represent good intentions without good consideration. With free weights the force you are working against is toward the earth, and you need to position yourself such that the movement you will be performing is along that same direct pull.
If you are using a different form of resistance, then the force you are working against can be different from the straight down pull of gravity. Resistance bands use elasticity to provide a variable resistance (it’s harder the further it is stretched) so positionally you would align your movement to be pulling the band into a greater stretch. This type of resistance is more intuitive because the band will stay slack if not pulled in such a way to stretch it.
A bit more confusing are cables. Cables provide a constant resistance but are typically connected to a variety of pulleys which means that the effective weight is not the same as the actual weight on the plate stack, and the resistance force is directly along the line of the cable. To ensure you are aligned correctly with cables, position to ensure your movement is along the direction of the cable that you are pulling.
Levers are perhaps the most challenging because it is not always clear how to position. T bars, landmines and leg curl machines are examples of levers used for resistance training and each is unique in how to best position. In general, if the machine has a pivot point then you want to adjust to align your joint with that point. And then ensure that your angle of movement when using the lever is using muscle, ideally the muscle that you are trying to work.
I know that it can sometimes be confusing to think about resistance force vectors, but taking a moment to consider the direction of force that you are working against can go a long way toward increasing the effectiveness of the time that you are dedicating to exercise.
And, as always, let me know how I can help.