Pick any good workout program. It doesn't matter if it's a pure bodybuilding program for mass, a pure strength program, or a training plan designed to get you ripped. Got it? Okay, here's the thing: you can get fat or lose fat on any of those workout plans. Your results will depend almost entirely on the foods you're eating.
Cars and Steering Wheels
Think about it like this: Training is the car. Diet is the steering wheel.
A muscle-building workout isn't going to take you anywhere unless you steer growth with supportive foods. A fat loss workout is going to stall out and leave you stranded if paired with an incompatible eating plan... unless you were a total couch potato before you started training, and even then you'll hit a wall if you're not steering.
What if you have no basic dietary principles? You know, like some general rules you follow when it comes to food, but not necessarily a "diet." Well, then no one is at the wheel of your car.
"Duh, That's Obvious!"
Is it? Diet is the hard part. Training – even those grueling, crap-a-kidney workouts – are at least satisfying and fulfilling. Not eating something you really want to eat doesn't exactly get your endorphins pumping. So we ignore the diet part. Or at least we try everything else first. We'd rather train harder than eat smarter, and that often backfires.
But Diet is Magical
A few better foods eaten consistently, a couple hundred fewer calories, a couple hundred MORE calories in some cases... abracadabra, results! And all without changing your training – assuming "training" means more to you than taking a slow walk on the treadmill.
Now, you can and should change your workouts if you have a specific goal in mind. But if you're training hard and being consistent and you still have a problem – too fat, too skinny, no muscle gain, strength stalled etc. – then it's very likely a diet issue.
Most people would rather not hear that because eating is emotional, habitual, social, and psychological as well as physiological. It's tough to get your nutrition plan right, and it takes time to master that side of the good-body equation. You'll have to crack a book, read plenty of T Nation articles, and – most importantly – experiment.
But the work is worth it. In fact, it's probably the most important aspect of fitness. Take the wheel and floor it.