What weight should I do when doing an exercise? How do I know when to increase the weight?
You have to remember that in order to stimulate muscle growth you have to push the muscles outside their comfort zone. For hypertrophy for example, that would mean doing usually 8-12 reps with the right amount of weight where you will struggle to complete the last two or three reps.
Since the strength level varies from person to person and from exercise to exercise you would need to find out for yourself what is the right amount of weight for you on any particular exercise. The best way is to start light and work your way up on the weight to the point where you will struggle to complete the last 2-3 reps of the designated number of reps on any given exercise. You should stay with that weight until it becomes easier to complete all the reps and only then you should increase the weight.