A few people have been giving terribad advice. At this point, you have changed your diet in such a way in which you are getting more calories than you are burning, which is why you gained the weight. So, if you want to get back to your original weight, you will need to eat fewer calories. In general, you will want to lose around 2 lbs a week (it might get slower as you go further along). Dont try and lose more because that is not healthy.

In general you

1) Track your calories. There is a phone app called myfitnesspal that makes it pretty easy to do. If you don't have a smartphone, they have a website. When you do this, you will learn that certain foods will have a shitton more calories than you realized, and this will cause you to have to change your diet. A general rule of thumb

i) Augmenting your food with veggies adds mass without the calories and it will add fiber and vitamins.

ii) Protein tends to fill you up. I use biochem protein powder due to its low calories. There are other things that are high protein. Experiment, and try things and find a few you like.

iii) Try to avoid sugary bullshit. Its a bunch of calories that will make you hungry in an hour.

iv) Make sure you count everything you eat, and measure your shit. Your eyeball tends to lie.

2) You will want to stay hydrated and drink plenty of water.

3) Cardio helps in the short run. You can burn 200 calories just by running for 20 min. But you don't have to do running, incline walking, swimming, biking are all great choices. If you start running, you may want to start with a progression program (like couch to 5k to avoid joint pain).

4) Lifting weights helps in the long run and it can make you look better when you get skinnier. You won't burn calories as quickly as when you do cardio, but you do burn calories just by being strong.

5) You CAN drink diet sodas (unlike what some people said), sometimes you need to extra caffeine, and that's okay, but it does dehydrate you.

6) You can have a cheat day, just don't go crazy.

7) It doesn't matter when you eat. Eat whenever you feel comfortable.

8) There is a bunch of bullshit diets out there. Do whatever you like, just make sure you like it and you are eating at a calorie deficit.

9) If you make a tuna sandwich with light mayo, you will need to add sweet relish. It makes it edible.

It will take a while, but as long as you are on a downward trend, you will eventually lose the weight and get back to where you were.