There's no way to answer that. You have to figure that out for yourself. It's all part of the process. It will become natural before long.
First thing to know is....."fuck respectable." There is no respectable. Respectable comes later. Don't start worrying about how much weight is on the bar.
Here's a guideline to start with: on each excercise, start with a weight you can lift comfortably, but still reasonably challenging, ten times. Concentrate on form, not how much you lift. Then, for arms and shoulders, add ten lbs and do eight reps. Then add ten lbs and do 6. For legs, back, do the same thing but add 20lbs each time.