Considering your age, get a check up done by a physician to avoid problems such as sudden rise in blood pressure, increased load on joints and the heart. Know your limit before you start.
1. You will need to follow High Resistance-Low Volume work out and do this initially under supervision. You can combine this with aerobic activity where you train yourself at roughly 60-80% of your heart rate.
2. High Resistance activities are running on an incline, stair climbing, step training and cycling. The muscles will have to work harder against higher resistance and this will give you a greater muscle size and mass.
3. Start with 500 grams weight since you are a beginner. When you can do 12-15 reps comfortably, then increase the weights.
4. Lift at a speed that you can control. On an average, each rep should last for 4 seconds: 2 for lifting and 2 for bringing it back.
5. Large muscles of the back, thighs and legs should be exercised first as they fatigue fast. The smaller muscles like the Biceps and Triceps should be exercised later.
6. While lifting weights remember to breathe easily.
7. Your body will take it’s own time to get adjusted to the routine of weight training. Don't compare yourself with others and don't get discouraged.
8. Training three times a week should be fine.
9. You should have a balanced diet of complex carbohydrates and proteins. Have a meal within an hour of your work-out.