Question: how to do reps in gym?
Answer: For fat loss: One to 3 sets of 10 to 12 reps using enough weight that you can only complete the desired reps.
To gain muscle: Three or more sets of 6 to 8 reps to fatigue. ... 
For health and endurance: One to 3 sets of 12 to 16 reps using enough weight that you can only complete the desired reps.