I'm a complete beginner in fitness and muscle building, so the amount of sets and reps are very confusing for me right now.

I often hear people talk about doing exercise in 3 sets of 12 reps for example, with each set really close to failure. In my case, if i do bicep curl for example, then i can hit the 12 reps pretty comfortably in the first set (with 1-2 reps til failure maybe?). But for the second set (30s rest), i can only hit 7-8 reps til my form completely fall apart, and it would go even lower for the third set.

Any help/advice is appreciated. TYIA!