although you can use weights to add resistance to your lunge, most people do not need that added resistance if they do lunges with good technique. a good technique requires a good level of power, flexibility, and stability but do not worry if you don’t have them fully since doing this exercise, again and again, is going to help you out with that. just remember to slow down to have more control over your reps.
the way your leg is positioned is the same for side lunge and back lunge, the only thing that differs is the other leg that’s on the air. to put it in simple terms, the more degree between your legs, the more outer muscles of your leg you use(gluteus, tensor fasciae latae, vastus lateralis, semitendinosus, and semimembranosus), and conversely, the less degree between your legs, the more inner muscles of your leg you use(adductor muscle group, gracilis muscle, vastus medialis, and biceps femoris).