I run cross country and track for college. I started in high school, was really weak at first, and is now a 4:30 miler, went to state championships, etc. I started somewhere and you have to, too.
How to begin?
It sounds like you're a beginner. I suggest you start with a run/jog/walk of 2 miles for a week. Every week, add a mile. So in a month and a half, you'll be up around 10k (meters)a day. My average mileage is 10+ miles a day, so I think your max should be around 6.
Biomechanics?
Run tall, don't hunch.
Pump your arms straight through at hip level. Your arms never cross your body.
Don't overstride. Don't land on your heels.
Land on the balls of your feet.
Be relaxed.
Don't shrug your shoulders.
Run everyday?
I do, since I'm a competitive runner. For you, I think you should try to run everyday with one day a week for rest or cross training.
Cross-training?
Try swimming, biking, lifting weights, elliptical trainer at the gym<<<very good, etc.
These are very general ideas from me. Give me more info about your situation and I can help you better. More advanced workouts i can give you are lactate-threshold workouts<<<<burns the fat, tempo runs, etc. Hope these work and tell me about your results.