How heavy should you lift?
Depnding on your current fitness goals and your individual abilities, each of your lifting sets will consist of approximately 10-20 reps. Keep in mind - there is no magic in lifting the certain number of reps. If you will just count to 10 reps for example and finish your set there because your fitness program is designed to perform that number of reps although you could do more - that will not be so much beneficial for you, The main idea is - it has to be very challenging for you to perform last given reps of set. Let's say, you have taken for your biceps curl dumbbells of 4kg and have to perform 15 reps. If you have performed 15 at the moment and feel that you still could do some more - do more. And then for your next set take heavier weight in order to not be able to do more than those designated 15 reps. And this will be your optimal lifting weight.
If you were not able to perform all desired reps of set, go for lighter weight next time so you are able to complete all your designated reps & sets.
What counts is not that certain number of reps itself, but the challenge you have to overcome while lifting. To load yourself up optimally means it is hard and challenging for you to finish your last lifts.
Correct form of exercise
You can lift as heavy as you can WITHOUT breaking correct form of the given exercise. If you, instead of curling just your biceps for example, are swinging all your upper body and using momentum, it will not be benficial for your muscle growth. Choose the resistance which allows you to perform the exercise correctly while keeping you challenged during your last reps. And yes, it has to be so hard.
Be not afraid of lifting heavy
Most of my new gym clients are very worried at the beginning that they could get too muscular by accident and that's why they avoid lifting heavy weights. Any long-term gym member will just smile at this idea:) Because they know how hard it is to grow your muscle, how much effort and dedication you must put in your workouts. Building muscle is hard long-term work and by any means it can not happen to anyone by accident.
Enjoy your lifts,