Arguably the most important thing to consider for anyone who's just started working out
, is pacing. You're not going to be able to do a hundred reps on the barbells on your first visit to the gym, you need to work your way up towards that. It may not seem like a lot of work, but ten to fifteen reps, for example, of moderate resistance is more than enough to stimulate the muscles enough into growing. As you continue then over the course of a week or two, you'll find yourself getting physically stronger, and this is where you would increase the weight, or add more reps. The same strategy applies to aerobic and stretching exercises. Start off at a light jog on the treadmill for 20 minutes before trying to run a marathon!