Progressive Overload focused. For some exercises, you will notice that the reps decrease. For these, you will increase the weight as your reps decrease. I recommend increasing by 2.5-5lbs for each additional set. Keep in mind you want to really be pushing yourself with weight without sacrificing proper form. The last 1-2 reps should be very challenging to complete, but not so challenging that you are unable to execute with good form.
For exercises that I recommend a range for the reps (10-12 reps) I recommend starting with a moderate weight and try increasing the weight each set. You want to aim to hit the higher end of the reps range (for this example that will be 12 reps), as you increase the weight, you may find that on the last set you are unable to complete all 12 reps with the heavier weight, and this is when you would drop down to 10 reps.