Situps are kinda worthless unless you're doing them to pass some sit-up test, but even then most freeweight core exercises will make you better at sit-ups than sit-ups would.
Machines are also pretty worthless, except maybe an assisted pullup/chinup machine if you can't do them on your own. I realize you've probably heard it before and I'm not any more likely to convince you, but consider that you can also finish in like a tenth of the time when you do freeweight exercises that with one exercise obsolete about half of your average 16-machine nautilus circuit.
Diet is also by FAR the biggest part of weight loss. Sorry to say. Running around the planet won't do you more good than not eating a cheeseburger - in fact, after a certain point, your body learns to cling lovingly to its fat to maintain it during long hauls. Keep cardio to within 30-60 minutes. Lift weights 2-3 times a week, and if it comes down between weights and cardio due to time, always go with weights - you burn less calories at first with it, but in the repairing process you'll burn a lot more.
How to lift weights:
And get your logbook here:
A million and one diet books, i recommend Lyle McDonald's rapid fat loss handbook:
Since you're just starting, you can easily do Starting Strength and cut calories at the same time.
Doing this, I lost over 27 pounds of bodyweight and gained about 9 pounds of muscle in 3 months (33.6% BF to 23.7% BF by tape measurements, calipers register me lower).
tl;dr SQUATS, DEADLIFTS, BENCH PRESS, AND CHINUPS + DIET = VICTOLY