I've got to agree with everything Aussie Digger has said. Something universal about military training is that you will learn that you have limitations which far exceed anything you have ever dreamed of and believe this, you will learn where they are. Keeping that in mind, stamina is without question the most crucial skill you could improve upon which will make your basic training "easier". I stress "easier" because it will be hard no matter what manner of shape your in. I advise everyone entering the service to take up running well before they go in, if you hate running so much you can't seem to do it on your own you are not going to like military life very much.
Aside from that my 2 biggest pieces of advice are thus:
1. Listen and learn what they are teaching you. Irrelevant of your country, your armed forces have probably trained millions of individuals before you. The methods they use work better than any method you may personally have used or feel more comfortable with. Weapons training being a fine example, most people can't shoot to save their life, but if you fire your weapon exactly as they tell you, you'll be quite the marksman.
2. Don't make the mistake of thinking about how long you have to go before you graduate. Conquer your training and all of the challenges it encompasses on a day by day basis. Forget about 12 weeks from now, concentrate on today.
Other than that, as long as YOU want to be there, nothing will stop you. Just remember that the day you earn the privelage of joining the ranks of your army, you will have accomplished an achievement very few do, and that is something that can never be taken away. Good Luck!