Calories In < Calories Out = Weight loss
The most important thing I would say is don't crash diet (Atkins diet etc) as that is not healthy. Make a lifestyle change and make it sustainable, else once you drop the weight and go back to your old habbits, the weight will come back real quick.
Just remember that weight loss does not necessarily mean just fat loss. You should aim to lose around 2lbs a week max, any more than that and you will be losing muscle mass as well as fat, and sure the number on the scales will drop but you won't be looking good by the end of it!
Best thing to do would be to calculate your maintenance calories, work out how many calories are burnt through the day e.g. how active you are, how much exercise you do, and deduct 20% to cut weight. Monitor results and adjust accordingly to keep within the 1-2lbs per week.
You should aim to try and eat about 1g of protein per lb of body mass each day. Slightly less for carbs and you also need to consume healthy fats e.g fish oils, seeds , nuts etc. Also to be drinking plenty of water.
To retain muscle mass whilst dieting you really need to be active and try and get your muscles working at least a few times a week.
If you have an Android or iPhone the "MyFitnessPal" app is really useful for tracking what you eat and weight changes etc.
EDIT:
I have found this online calculator can be very useful for working out calorie requirements etc.
http://www.1percentedge.com/ifcalc/
Although related to Intermittent Fasting, you don't have to be doing that to use this.
