Honestly, I don't know if you have an physical limitations or if you have any allergies... But let's have a go. :)
1. Diet. And by this, I don't mean "drink a shake for 30 days and be small"... I mean lifestyle change. Do you eat 3 meals a day? If yes, good. If no, start. Let's assume you do already eat 3 meals a day, what exactly do they consist of? How big are they? If you eat cereal for breakfast, try switching to a healthier brand (ie. Kashi). Fiber, proteins... the good stuff that fills you up. At lunch, try not to buy food, instead you should pack a light sandwich, yogurt, and fruit. Eat and do something else at the same time. Take your time eating because we can eat a lot more than we need to most of the time; our brain doesn't register that we're full as fast as we can eat. Drink your water rather than sodas. And for dinner, remember to pack in your veggies and fruits. If you plan to be a fighter, I will assume you're not against the idea of working out.
2. The working out. If you can afford it, go to a bootcamp class at a local MMA gym. Check out local Karate, MMA, Self-Defense, etc places. Check prices. I recommend taking a basic class (you can usually find free trials/one day courses).... that way you can learn how to properly stand, hit, kick, etc. You don't want to develop bad habits as a fighter. Once you get the basics, if you can't afford to continue, try to work something out with the owner. Usually local shops will try to help you with your situation. However, assuming that they don't help... and you still don't have that much money, check out a local community college's PE classes. The ones near me have stuff like bootcamp and etc. These courses tend to be 1 unit and that means you only have to pay for that 1 unit. But let's assume the worst case scenario, you have no cash. You at least have the internet. Look for free workout videos. I suggest working out on your own, rather than training on your own without ever being properly trained. Be properly trained on how to fight at a gym. However that being said, it is still possible to teach yourself how to fight from the comfort of your own home... it's just not recommended.
There's so much on this topic, that I could write you a novel... but this is just some quick advice. Good luck. If you need help, contact me. :)