I've been trying to gain a six pack for a long time... with mixed success. Weight loss, muscle building is not a once off summer course you take- but really a lifestyle that you should adopt - the earlier the better. The longer you put it off - the worse it gets.
I must say that going to the Army helped me a fair bit. I was about 80kg and relatively unfit when I got in. I couldn't even do 3 pull ups. But after the 3 months of basic military training, I really made a huge improvement. I could do 9 pulls ups. I found I could run faster than most of the people in my group including some who were more stronger than me.
However, the training was too tough (for me). It was just too much too soon. It was only after spending 1 month in hospital due to chicken pox, and 1 month rest - that I made better physical progress. IIRC I think the fastest I did my 2.4km was in 7 minutes 30 seconds. I could also haul up 13 perfect pull ups. Pity I didn't keep it up. Now I can't do 3 pull ups. :(
The main thing is to train consistently. Don't wait until Saturday to do a big 10km run. Try and do regular runs - 1 to 2 km a day. You also have to maintain a good running posture - y'know don't slouch, don't drag your feet, it doesn't look cool and its not healthy.
If you can't do running, then swimming or some other aerobic activity that gets your heart pumping steadily for 20 minutes - 40.
Running or exercising alone can be a very boring experience. So try and get another friend or two to go exercising with you. Hopefully you'll both encourage each other to keep on going - choose wisely tho, or you'll end up sitting on the couch with your friends, drinking beer, eating chips, and watching women's tennis or something dreadful.
Above all else, try and eat healthy stuff. You are what you eat. Drink water not coke. Eat plenty of fruits. Eat plenty of fibre - sweet potato, brown rice etc.. And avoid white bread, white rice, fried food, and other processed food. Lately, I've been eating rolled oat porridge for my breakfast and secondary meals/snacks. No sugar, no salt. In contrast to my younger days, I don't need to put sugar, honey, or any sweetener into it. I think once you start "doing" without sugar or sweets, your level of intolerance for "bland" food decreases.
You need to motivate yourself too - so surround yourself with plenty of positive exercising stuff- like running magazines, health magazines, etc..
And try visiting good exercise sites on the internet - like this one - no gimmicks, no hype