First off, you can't spot reduce. If you lose fat, it will be overall fat. Since it sounds like most of the fat accumulated at the stomach area, it will probably be the least noticeable area when you start to lose body fat. The stomach area is a common place most people want to lose fat, but it’s also a common way for most people to give up their exercise program simply because it’s the hardest place to lose overall. The best advice is to not look at the stomach fat and concentrate on keeping up with a exercise and diet program. It's hard to say how long, and since it sounds like you haven't kept up with any program, it may take up to a year or two to get what you want.
Second, asking if doing X or Y will help you lose weight is like asking if you reached for the moon will you be able to touch it. There is no way in knowing if you will in fact lose weight since it depends on so many factor. Doing anything is better than nothing. However, if you have a specific goal in mind, such as getting a "the abs" then you will have to have a specific exercise program and diet that fits around it. If you're simply looking to just become healthy without anything specific, then you can pretty much just look at your current diet and physical activities, and sub out the things *you* consider bad, and sub in things *you* consider good.
A better way of asking this question would be:
If I did X or Y for this duration at this intensity at this many interval, will that be considered good? For example, I want to run on the treadmill at 5.5 mph for 30 minutes at least 3 times a week.
If I substitute Y for X in my diet, will that be considered good? For example, I normally drink 5 cans of coke throughout the day. I want to quit by reducing it to 3 cans this month, then hopefully replacing it down to one or two cans by the next month... will this be a reasonable goal?
