In general, use enough resistance to fatigue your muscles when you have completed the appropriate number of repetitions within a set. If you cannot complete all the repetitions within a set, decrease the amount of resistance. Always make sure the last repetition is as pretty as the first. Try to complete 1 set of 15-20 repetitions. To really see results, you must decrease your body fat.
Many people confuse body fat with obesity.
Body fat is the percentage of fat that you store in your body as opposed to lean muscle mass. There are very thin people who have too much body fat because they do not exercise. There are also average to above average sized people who have very little body fat and are all muscle. At this fitness level, if you e xercise aerobically 3 times per week for at least 20 minutes , you will see more results from your strength training. In addition to aerobic exercise, you should also do some form of stretching.