Take one to two days off between working out a specific muscle group - so this normally means a total of 3 workouts a week (or 4 times, if you do a routine where you split the workout into upper/lower body routines) Regularity is vital - once or twice a week will not accomplish much; neither will 2 or 3 days in a row on the same muscles with weights!
Think of it like a spectrum. At one end is aerobics, the other, weights, and they overlap in the middle. Aerobics is designed to primarily work the heart and lungs, and the muscles secondarily. Weightlifting is the reverse. Each pound of muscle uses, by itself, about 50 calories per day - even when you are not exercising (See Duke University's research for evidence of this). Since 1 pound of fat is equal to about 3,500 calories, if you gain about 5 pounds of muscles (which is more dense, small than 5 pounds of fat), that means you'll lose a pound of fat ever 2 weeks in ADDITION to those your burn off WHILE exercising! This is why weightlifting brings about changes in appearance, muscle tone and strength faster and to a greater degree than aerobics. Aerobics (James Fixx, a USAF doctor is generally credited with the term) also called "cardio workouts" is designed to work the heart and lungs primarily, not the muscles.
"If you are only doing aerobics, you are missing the boat," says Wayne Westcott, co-author of Specialized Strength Training. "You will lose muscle mass and your metabolism will slow down as you age if you don't strength train." See the full USA Today article
A repetition or rep is each time you lift a weight through one range of motion and back to the starting position. A set is a group of reps separated by a rest period of 3 minutes or more.
3 to 5 sets per exercise per workout seem to give the best benefit.
One of the best workouts is to do 4 sets of each exercise; starting with a set of 12 repetitions, then a set of 10 reps, then 8 reps then 6 reps. Start with enough weight so you can do 12 reps, but NOT 14 or more, the last rep in each set should be tough! Then ADD weight so you can only do 10 reps, then add weight to 8 reps, add again so you can only do 6 reps. Remember, once you start to do more than 12 or so repetitions in one set, the weight is too light to effectively work and change the muscles.
Put enough weight on each machine or barbell so that you can NOT do more than 12 repetitions (reps) at one go (set). This is important - do not worry, you won't get big! 99% of the people you see who are unnaturally big (both men and women) got that way by taking steroids! So don't take steroids!!!! The American College of Sports Medicine (ACSM) recommends that most people complete two or three sets of 8-12 repetitions of each exercise for maximum strength results. (This means you should feel pretty fatigued by rep number five or six and be struggling to lift that last repetition.) However, if you have been sedentary or are over 50 years old, only two sets of 10-15 repetitions at a lighter weight may be more appropriate.
In each workout, start with larger muscles (chest, back, thighs) and work toward smaller muscle exercises (shoulders, arms, calves, etc.).
Move as slowly as you can through a full range of motion - except that you never should "lock" a joint. Stop before the joint is straightened completely and "locks".
Slower movement reduces the risk of injury and ensures that each part of the muscle is worked fully and evenly. When you go fast, you use the strong parts of the muscle to accelerate the weight to coast through the weaker areas. That's cheating and hey, if you cheat... you only cheat... yerself! :-)
For substantial health benefits, adults need to do at least • 2 hours and 30 minutes (150 minutes) each week of moderate-intensity* aerobic activity, OR • 1 hour and 15 minutes (75 minutes) each week of vigorous-intensity* aerobic activity, OR • An equivalent mix of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed for at least 10 minutes at a time, preferably, spread throughout the week.