I used to struggle with this same dilemna. I can tell you a few things on what to do to increase fitness, but it all comes down to having the desire to go out and actually do it that will make the difference.
First off, you have to start eating healthier. You need to drink between two and four litres of water a day, just to keep you constantly hydrated. You need to eat a lot of pastas and breads, and stay WAY away from sodas - the carbonated water cuts down on your wind and makes you not be able to run for as long or work as hard.
Second, you have to work out. Football is a task that involves more than just one part of the body, as you constantly have to use your chest, head, legs, arms, feet, hands, everything. Stamina needs to be high in the whole body, so you need to be strong everywhere.
Third, RUN. Don't think you can go around this: you have to run to build stamina. Go out and run at least every other night, to the point where you can run two or three miles at a good pace without stopping. If you cannot do this, go out and just jog a mile, then walk back to your starting point. Building stamina is a gradual process, and you have to do a lot of work to gain it.
Another thing that will help with your stamina being built up is playing games. You have to learn how to last a full length game running non stop, so playing games will help you learn how to do this. Like myself, I'm one of the slowest players in my teams outside of games, but when I cross the white lines, I have a different level of fitness that enables me to play full matches.
A last tidbit of advice I give it to hustle at practices or when you're training with a few mates. When you're shagging a ball you shot and missed with, jog to it, and jog it back. When you're passing the ball around, move, make little runs. You might not notice it, but your fitness is picking up if you're always moving and always running.