Nutrition and exercise go hand in hand if you want to maximize the effects of both on your body. If you do one over the other, you'll certainly see some results, but if you do both you'll really see some amazing transformations.
Ironically, keeping up with your nutrition is a harder step to overcome than exercise. Eating will isn't cheap, eating well takes planning, and eating well isn't always convenient. Sticking to it will not only help you reach your health and fitness goals faster, it'll give you more energy too.
Jumping right in to good nutrition from bad can be similar to quitting smoking cold turkey, especially if you're bad at committing to things. If this is the case for you, every 2-3 days replace one bad eating habit with a good one. In less than two weeks you can be making healthier decisions and start seeing all your hard work at the gym really shine through.