I weigh around 180 pounds and i need too loose around 40 pounds in three months . can u guide me as to what i should do?

Start with lifestyle: Takes more than 3 months! try changing dietary habits. Reduce calories eaten. Choose healthy foods and watch your calories. Make sure you watch what you're drinking too (soda, alcohol, etc). Exercise several times a week. You need to burn more calories than you consume. There are several "apps" that will track the information for you. Beyond that, you can consult your physician for other options.

Rapid loss bounces: Of course, you can lose a lot of weight with appetite suppression medication, extreme exercise, restriction diets, and so forth; but in the big picture, all of that weight will bounce back after your event in 3 months. Do this only this one time, and then get serious about a lifetime plan of maintenance weight loss to get to your goal weight and stay there.