"Abs are made in the kitchen" as the internet meme says.
The most important thing is to watch your calories. I find it all a bit fiddley, but apps like my fitness pal help a lot with calorie counting.
On top of that focus on cardio exercise; running, rowing, biking, skipping (jumping rope) etc etc.
Going to a gym 3 times a week isn't that effective if you are sitting all day and watching 5 hours of tv a night. Try and be more active generally. Cycle or walk places, things like that, as well as doing your gym sessions.
I think one of the best things to do is find a sport that will get you out of breath. If you go to a gym just to lose weight, you can rapidly become bored - The motivation is not always there. If you pick up a sport that you enjoy, you will be eager to go and practice and possibly stick with the hobby longer and then lose weight as a side effect.