Eat right. Work out. Rest. Rinse and repeat.
First law of thermodynamics... you need to expend more calories than you consume to lose weight.
To figure out your caloric need, use the Basal metabolic equation:
(6.23 x weight in lbs) + (12.7 x height in inches) - (6.8 x age) + 66 = Basal metabolic rate (or resting metabolic rate).
This is the number of calories your body burns on a daily basis without any added activity.
Take that number you calculated and multiply it by the Harris-Bennedict coefficient (this formula takes into account how active your lifestyle is):
Sedentary: BMR x 1.2
Slightly active: BMR x 1.375
Moderately active: BMR x 1.55
Active: BMR x 1.725
Extremely active: BMR x 1.9
Finally subtract 500 from that last number you calculated, and that will tell you how many calories you should be consuming on a daily basis to lose weight (conversely, to gain weight add 500-1000 calories).
Make sure the calories you ingest are "clean." That means healthful foods: lean protein, good carbs (brown rice, whole wheat pasta, steel cut oats, et cetera), good fats (olive oil, avocado, Omega-3s), veggies, and fruit. Also, this should be broken down into about 40% carbs, 40% protein, and 20% fat. No soda, corn syrup, fast food, heavily processed foods, high sodium... you know the drill.
Weight train 4 days a week, because building muscle will increase your metabolism. Split your routine into muscle groups with two days devoted to cardio (exempli gratia, Sunday: legs, Mon: chest/biceps, Tues: 20-30 minutes cardio, Wed: back/triceps, Thurs: shoulders/abs, Fri: 20-30 minutes cardio, Sat: rest). Bodybuilding.com
is a great place to get started if you need workout regimens or examples of proper form/exercises.
Lastly sleep is important, because that is when your body builds muscle. Make sure you get a good rest each night.
That's all there is to it.