I'm by no means an expert, but I've also had to modify my diet considerably in the past months. My old man started having health issues (high blood pressure, cholesterol and even sugar issues), and so as to be all together in the same ship, we all adopted a similar diet.
Before I go further, I'm also 26,weigh the same and are about the same size. But instead of having a physically demanding job, I'm stuck in an office.
I also used to have a lot of fast food meals and drank a lot of sodas and sugary water. But since my dad went on a diet, I'm trying to have the same home cooked meals as him: We have kept off flours (in my case, we've switched to corn tortillas or whole weat bread), ditched refined sugar (relying on brown sugar and splenda/fructose for what must be sweetened), ditched deep fried foods and barely eat any cheese or butter, and drink water, or fresh juice. I also try to have a nice serving of veggies/fruits with every meal and eat an apple or a pear a day. For protein we eat red meat, chicken, fish (a lot of salmon; tuna is also good but I'm allergic to it), egg whites and a bit of ostrich meat.
That plus excercise (1hr walking/jogging/running a day) has made me loose 20lb in about 3 months, and in my dad's case halved his blood/cholesterol count (tough he also took some medication to help) and his blood pressure and sugar are now spot on. A funny side effect I've noticed (and my mom has too) is that now my sense of tase and smell is sharper; it's like it was saturated with a lot of junk and once you take that away, you find a lot of flavors where you didn't notice them before.
Now, I'm the one who sticks a bit less to the diet, since when I go out with my friends its kind of hard to always find something healthy to eat, but just start counting calories and you'll start finding you'll make better choices. Do your reaserch cause sometimes you can have unexpected results (a very crude example would be a large hotdog at costco has 510 calories, while their Ceasar salad has....740).
What ever modifications you do to your diet, the key is making a plan and sticking with it!