@ladydenise226 In the weight lifting world, there is a thing called "training to failure" and "reps in reserve".
Training to failure means you do an exercise until you are unable to make a single additional rep without compromising your technique.
Your muscles are so tired that you physically cant do the lift the way you are supposed to.
Reps in reserve means you stop a certain number of reps before reaching the failure point as described above.
Training to failure is universally agreed to be the most muscle growing way to do a single training session, however it makes you fatigued for several days after and it is more beneficial to train more often, so people generally train with 2-3 reps in reserve.
So how to pick your weights?
Simple you do which ever weight makes it so you reach your desired reps with about 2 reps in reserve.
Generally for growing muscles you want to aim for between about 8-12 reps, in which case the weight you pick should be too heavy to lift 15 times (with correct form).
For example, if you were physically unable to do 15 squats in a row with 7.5lbs, then 7.5lbs would be the correct weight for you.
And yes, this is supposed to feel hard!
The lest few reps you should be really pushing your self to complete.
Part of training is also to train your will power.