As a guy who's actually LOST the amount of weight you're looking at, let me tell you what to do.
First of all; diet.
Breakfast: Grapefruit, maybe a cup of coffee. That's it. Welcome to hell, fatty.
Lunch: Ramen noodles or failing that, 2 or 3 bean burritos from Taco Bell, depending on microwave access/taco bell access
Dinner: Well, you've starved yourself all day... now it's time for lentils and rice/beans and rice. Emphasis on the beans or lentils. Lentils lentils lentils. I can't say it enough. LENTILS. Think about a quarter pound of ground turkey or chicken breast, lentils, and rice. More lentils, less rice. Lentils and rice run about 79 cents a pound respectively, and it's an INSANELY healthy and economical combination.
Well, you've got a gymn membership! YAY!!!
Now use that motherfucker.
There's an aerobic machine called an "elliptical machine" This machine is your bitch now. 30 minutes a day, five days a week. If you're not sweating, you're not doing it right. If you get comfortable on it, INCREASE THE RESISTANCE. I currently have the machine I use maxed out on tension. Ellipticals are a real monkey fucker, too, so if you can't handle it, start on a treadmill you pansy. Ellipticals are MUCH more efficient at burning calories; this is the machine you want to end up on.
Weights. Look, without weights, you're not going to get lean muscle mass, and without lean muscle mass, your metabolism is ALWAYS going to be shit. Here's the rules:
1 start with benchpress. it's a good all arounder. Set the weight to something you can easily push off the bar
2 pump it as many times as you can, stopping at 15. If you can press it fifteen times without straining, increase your weight
3 10 presses is the happy medium. You can get the job done in as little as six presses, but ONLY if you're barely able to push up number six. If you can, use a spotter. When I say "straining to push up" I'm not shitting you. You should come across times when you can't push the bar off your chest if you're doing it right; MAKE SURE TO HAVE A SPOTTER!!!
4 Three sets of presses (that means press the weight as many times as you can once, rest, repeat 3x)
Those rules apply for curls as well. You should alternate curls and presses, working your biceps and chest respectively, letting one muscle group rest while you work the other. Three sets of each per workout.
So, open, 30 minutes aerobic on either treadmill or elliptical, alternating three sets of bench presses and curls. The aerobic will automatically get your legs, and being fat, you've already got strong legs.
I lost a lot weight this way, and my whole family are fat as houses. Remember, if your natural build is large, you'll always have a certain minimum weight, and the only choice you get is whether to make that muscle or fat.