Back in the day, there were no personal trainers. If you needed to learn how to exercise, you got a subscription to one of a few well-known "muscle magazines" and read several issues from cover to cover. Then you joined a "Y" and began to discreetly observe what was going in the weight room, trying to match up what you had read in the magazine with what you were seeing in the gym. Eventually, you put together a series of exercises, sets, and reps that worked for you. Back then, any strength training program you developed would be strictly based on a seat-of-the-pants approach. You learned by trial and error.
Today there is a vast body of scientific literature focused on the various benefits of numerous forms and types of exercise.1 However, scientific studies are not good at evaluating the how-to's of getting fit. Fortunately many informal resources are available, all intended to point you in the right direction. But not all of these resources are accurate or trustworthy, and the challenge is to identify a set of basic principles that will be applicable to your specific situation.
Firstly, before getting started you need to make sure that it's OK to actually get started. Let your doctor (your family chiropractor, family physician, or internist) know what you're planning to do and have her tell you what you need to watch out for, if anything. Next, you need to make a commitment. Consistency is the key to deriving lasting value from exercise. Additionally, irregular exercise sessions will often lead to injury. If you're serious about getting fit, then make a commitment to yourself to participate in a 12-week program. At the end of 12 weeks, you'll evaluate how you feel, what you've accomplished, and whether you want to keep going.
In terms of strength training (that is, weight lifting), three sessions per week is ideal. By doing "split routines" you can exercise all the major muscle groups each week. On one day you'll do exercises for the chest and back. Another day you'll do exercises for the legs. On the third day you'll focus on the shoulders, biceps, and triceps. This set of split routines will produce optimal results for many people.
Importantly, you'll be doing chest and triceps (and back and biceps) on different days, thus avoiding the potential for overwork and injury. But you may find that an alternate set of split routines works best for you. The key is to start slowly and build up strength gradually. Once you have some experience and an improved level of fitness, you may branch out and vary your basic routine, experimenting and seeing what works best for you. In terms of sets and repetitions (reps), three sets per exercise and eight to 12 repetitions per set represent the classical, tried and true method of getting fit and making gradual strength gains over time. For any strength training exercise, start with a weight at which you can do eight repetitions comfortably. This should be neither too easy, nor too difficult. Of course, it's far better to err on the side of caution. You never want to do too much too soon.
As you go along in your fitness program, you'll add core exercise routines2 and aerobics exercise such as walking, swimming, biking, and running. If you work out slowly and gradually and maintain consistency, you'll have a great deal of fun and gain substantially improved levels of health and well-being.3
1Storer TW, et al: Effect of supervised, periodized exercise training vs. self-directed training on lean body mass and other fitness variables in health club members. J Strength Cond Res 28(7):1995-2006, 2014
2Kahle N, Tevald MA: Core muscle strengthening's improvement of balance performance in community-dwelling older adults: a pilot study. J Aging Phys Act 22(1):65-73, 2014
3Huffman KM, et al: Metabolite signatures of exercise training in human skeletal muscle relate to mitochondrial remodelling and cardiometabolic fitness. Diabetologia 2014 Aug 5. [Epub ahead of print]