First you need to find how much is the heaviest you can lift. What is the heaviest weight you can hold while doing one bicep curl? How about one chest press, one shoulder press? When you have those numbers, you want to choose weights for your actual workouts that’s at 60 to 70 percent of the maximum weight.
For example, if the heaviest weight you can lift is 15 pounds while doing one rep of a certain exercise, then you should probably go for a 10 pound weight for that particular exercise on the regular. For a good development of strength and muscular endurance, you want to get in at least 8 to 15 reps each set of exercise.
Remember that when you go over the 20 rep range however, you hit a point of diminishing returns, because most likely your form will be compromised. I recommend 3 sets of 10 reps for each exercise.
Know that your strength will increase with time and you should be upping your weights around the three month mark.