As long as you're being safe, and your form is fantastic, the answer is pretty simple and makes common sense.
For weighted exercises... choose a challenging weight that allows you to reach the lower end of the 'set range', with good form... and is challenging for you to reach the higher end of the 'set range'... if you can reach it at all!
For example... using 8-12 reps as our set... you want a weight that you can do at least 8 quality reps with. So, if you cannot do at least 8 quality reps, the weight is too heavy.
And, if you can do more than 12, with good form, the weight is too light!! ... next time, add 5 or 10 lbs.
Time to sweat!