For each of the exercises listed in your program, you will have a recommended repetition range (e.g. 6-10 reps, 8-12 reps, etc.). The weight that you select for that exercise should allow you to at least do the lower end of the rep range (e.g. 6 or 8) with proper form, but no more than the upper end of the rep range (e.g. 10 or 12).
If you find in your first set that you weren’t able to do the minimum rep range, then the weight was too heavy and for the next set, you should adjust the weight so that it’s less.
If you find in your first set that you were able to do more than the upper rep range, then the weight was too light and for the next set, you should adjust the weight so that it’s more.