I want to loose 100 pounds

Exercise is great. I would start by doing what you like to do, whether that's walking or something in the gym. Any movement is good movement.

For weight loss, you'll need to consume fewer calories than your body uses. Exercise by itself won't lead to weight loss if you end up eating the same amount or more calories than your body uses.

A good place to start is by logging your food without making any changes. Get a food scale to weigh solid foods and get used to logging. After a week or two, look back through your log and see where you might eat smaller portions or maybe substitute some calorie dense foods that you like with other foods that you also like but that have fewer calories. Make small, sustainable changes. IMO, the closer you stick to your normal way of eating, the more likely you are to stick with your reduced calories.