I'm somewhere around 5'8 and 138.8 pounds.
So this is how I got from about 160 lb to around 140 lb in about 4-5 months:
- Decrease calorific intake by 10% to 20%, I suggest to not do more than that.
I personally don't count calories but when I'm on a diet I just look at how much I usually eat and take a little bit off the plate
- Increase active physical activity by at least 30 minutes to an hour. For faster result do more, but don't overdo it. Swimming, running, stuff like that.
- Sleep
- Eat healthier foods (whole grain, vegetables, fruits, etc.)