If you want to get rid of loose skin then cardio is your best friend. Run, run, run. If you want a six pack and/or tone then run and work out with weights.
Traditional wisdom goes something like this----
You do have to reduce your calories,
Each pound is worth 3500 calories - so you have to cut about 3500 calories from your diet by the end of each week (that's about 500 less per day).
Make sure that if you are feeling hungry, you have a bottle of water nearby that you can chug. Also - eat as many veggies as you can to accompany your lunch or dinner. Lastly, eat food that is high in protein (maybe a 10 gram protein bar) to fill you up and keep you full between meals. Don't cut dairy (you need it) and make sure you get rest, too.
That diet plan will work if you can stick to