There's a chart that floats around the academic and internet worlds which is supposed to answer the question of “how should I lift for different goals?” It usually looks something like the one below, though sometimes it will have a Russian's name on it, because everyone knows all the true genius of fitness lies in the Eastern Bloc, just look at Putin with his shirt off.
For almost all lifters, this chart is a load of old bollocks. It refers to experienced lifters. For beginners – where a 'beginner” is someone who, given good rest and food, can progress from one workout to the next, and almost all people in gyms are beginners - the reps and sets aren't so important, and certainly the tempo of the lifts don't matter much at all. For example, it's been shown that the number of sets you do doesn't matter much.
The body changes because you ask it to do more than it did before. As a beginner, before you were doing nothing, now you are doing something, something is more than nothing, so your body adapts - you get stronger, and if you change your diet, you will get overall bigger or smaller.
Exactly what that something is that you do isn't that big a deal. For example, even Zumba or riding a bicycle would improve the barbell squat strength of a previously sedentary person - because in each case your legs are doing more than they did before, which was nothing. Much the same applies for other fitness goals, by the way; a sedentary person just lifting weights will improve their cardiovascular fitness, and just doing jazzercise will improve their strength.
For the beginner, the reason to choose a particular rep range or series of exercises is not that they're better than something else at improving their cardiovascular fitness, strength, muscular size or whatever - rather the person is laying the foundations for future training.
For example, let's say that today you can squat the bar, and after six months of BodyPump doing sloppy half squats while hating the fitness instructor for her cellulite-free thighs you find that you can do a good form deep squat with 50kg. But your friend just started with the 20kg barbell and worked her way up to squatting 50kg. Her squat would be technically better than yours because she's got more practice in the movement. And she would reach a 75kg squat before you did.
The important thing for a beginner is to learn the basic movements of squat, push, pull, and so on - and to practice them regularly, trying to do lift more weight, or the same weight more times, in each session. Just 1kg or 1 rep is more, and you will force your body to adapt.
The exact sets and reps aren't important. Obviously lifting weights you can lift for less than 3 reps means you are likely to be sloppy in your movement, more than 20 reps is usually not more of a stress than 20 or less; but outside those extremes, most will work just as well for beginners. The important thing is that consistent effort over time gets results.
How many sets and reps should I do? More than you did before. That's what beginners need to do. I know everyone reading this is advanced, honest. I mean those other people.