Congratulations on adding weight training to your fitness routine. As I’m sure you know, there are many benefits to resistance training such as improved body composition, increased bone density, and increased resting metabolic rate. For resistance training to be effective it should be included at least twice during your weekly exercise routine as a part of a healthy lifestyle.
Before adding resistance to your exercises it is important to develop stability and mobility to avoid injury and to be sure your body is prepared for the added stress of load bearing exercises. To find an ACE professional near you that can help you in this process, go here: https://www.acefitness.org/acefit/locate-trainer/.
The answer to your question regarding repetitions and load, the answer can be determined by your goal as it relates to resistance training. If your goal is to improve muscular endurance, then anything greater than 12 repetitions is an appropriate amount. Other goals associated with resistance training include increasing power, improving strength, and hypertrophy (increasing muscle size). These goals are associated with repetitions of less than or equal to 6 for strength, 6-12 for hypertrophy, and 3-5 for power (ACE Personal Trainer Manual, 2010; pg 321).
After you have determined your specific goal, you can move on to determining the weight that would be appropriate for each exercise you select. For muscular endurance and general health, 60-70% of max resistance is usually appropriate for 12+ repetitions. For example, if your max squat is 100lbs, then you would use 60-70lbs for your sets of 12 or more to improve muscular endurance and general fitness. However, being that you are new to resistance training; you may not know your maximal weight for given exercises. If this is the case, you can use your own RPE (Rate of Perceived Exertion) as a way to know if they weight you have selected is correct. You should feel that your last set of exercises, for whatever repetition range you choose, is the maximal number of reps you can complete. In other words, if you are doing 3 sets of 12 repetitions for a leg press, at the end of your third set, numbers 10, 11, and 12 should be very difficult and you should not feel that you can complete another set. Here is a link to the ‘Weight Training Load Calculator’, as well. http://www.acefitness.org/acefit/healthy_living_tools_content.aspx?id=8.