Go you for knowing you need to change!
I do the same thing - I say right, tomorrow's the day, then I don't do a thing right! Well, todays the day, so I'd better make the most of it!
I think everyone else's ideas a really good. I just wanna add a few I thought of.
You said money is tight. I find sometimes that can actually be helpful. I'm an on campus uni student during the year, and when I go to classes I always try and make sure I take no money with me. If I have no money, I can't spend it, and if I can't spend it, I can't buy crap. I have to walk back to college to eat. Maybe you could try that? Try taking no money with you, but do take something healthy to eat. That way, you can't choose the junk food, cos you have no way of buying it. You have to choose the healthy one. A step on from this, as you get good at it, you could start putting the money you save into a jar, and after a week or 2 weeks or whatever, take it out and buy yourself a present, like a new DVD or a necklace or something. Reward yourself.
Another one that has sort of already been said is trying to do everything all at once is too hard. It can't be done! So instead, try smaller things. Say right, this week, I am going to get a small quarter pounder meal rather than a large one. Don't do anything else, just try and do that for a whole week. Next week, say right, I'm going to get a small quarter pounder meal with diet coke instead of regular. Try and do that. Just make really little changes, and when you succeed, reward yourself, you deserve a prize! Small changes may feel like nothing, but it's incremental. Every small change paves the way for bigger changes. And a few small changes = a big change. Small changes are easier, you're less likely to fail, and less likely to give up. When I go to the vending machine, I try and only buy a drink, not a drink and a chocolate. Or when I have a milkshake, I try and get skim milk. Little things can make a difference.
Star charts are also good. It's strange how much a gold star can make you want to do something. Find a small goal, and at the end of the day, when you've managed it, give yourself a star. When you have enough stars, reward yourself.
Those are my thoughts at the moment. See how you go. You CAN do it, don't let yourself say that you can't (though sometimes it will seem that way!). Because you CAN.