Avoid these common mistakes:
Not stretching. Get someone to show you how to warm up properly. Work on all the big muscle groups.
Lifting weights that are too heavy. Your back is the best barometer here. If it's bending, shift to a lighter weight and slow down your repetitions.
Using momentum to lift weights. Eliminate momentum from your reps by trying some reps at an agonising slowness. Try going up for two counts and down for four.
Too much, too soon: You start out with the best intentions, spend 40 minutes on the step machine and a barely able to walk the next day, let alone work out. It's human, but still ill-advised. If you're out of shape, increase the time of your workout or the weights you lift by 10 percent per week. No more.
Dehydrating: Your body needs water. Carry a big bottle with you and slug at it constantly. It'll help your lose weight. Don't worry too much about the high tech sports drinks. A pinch of salt in the water bottle and as banana before the workout will do the job just as well.