EAT GOOD. Depending on how pozzed your army is, they will run you through lots and lots of physical exercise. If you don't eat and drink (and chew) properly you will lose muscle and stamina. If you are smoking: stop now. Never touch a cig again. They kill your stamina and if you are still young you can recover most of it very quickly.
When there is a day where you got nothing to do (which will come sooner or later) it is best to do one of three things: Clean your room/equipment; fresh up on theoretical knowledge ; exercise.
First one is obligatory. Clean gear is functioning gear, and a soldier with clean and functioning gear not only looks good (to his supperiors as well), but will have an easier time in the field.
Second one is nice to have. Being able to correctly recite the exit velocity of a round from your standard issue rifle always looks and feels good. It also gives you something to stimulate your mind somewhat. You can also brush up on the history of your army/country. Trust me, your mind will wither if you don't use it, which will be most of the time.
(Learning how to identify tanks, vehicles and weapons of opposing nations is also a nice way to spend an afternoon)
Third one is an absolute must. You earn money by renting your body to your government. You better stay healthy or you may find yourself in a bad situation sooner or later.
You can always ask your superior forna task if you are really desperate. There is nothing wrong with slacking off every once in a while, but don't become one of those guys who desire nothing more but the couch in their life.
Socialize with your comrades. They are more than your collegues after all. You will be surrounded by them nearly 24/7, stay on their good side, offer help and advise, have a beer now and then. Nobody demands that you become best buddies, or even friends, but it bears lots of advantages.
Don't be the weirdo who can't undress in front of the others, but don't be the weirdo who does so without anyone asking for it either. Never masturbate with other men in the same room. This shit will get you bullied in the not cute way.
Even if most women are all right, don't ever start anything or enter their room unanounced. Even if they are totally OK with it and consider anything that happened an accident, they can accidentally ruin your career by making a comment to someone else, who then reports it.
Take half a second to think before you reply. Nothing is worse than "PRIVATE ANON, WHY IS THERE SPAGHETTI ON THE FLOOR?" and you just stand there babbling "Uh…Pants…um…pockets…uh…mother!"
Half a second of thinking won't look strange at all if you use the time to breathe in and straighten yourself for an answer.
Leave out unnecessary info. If you are asked why there is Spaghetti on the floor, don't start with how it got into your pocket, start with "It fell out of my pockets". If they want to know more they will ask for more.
If someone tells you to do something, do it not as fast as you can, but as thouroughly. There is nothing worse than being told to clean a room, and having the guy who inspects it unsatisfied with your work.
Practice drawing maps a little. Just tiny sketched are enough. Highly usefull for marches and figuring out which way to go later on.
When receiving orders, don't be afraid to pull out pen and paper. Not only does it look good, but it's better to do everything right once, instead of doing it wrong twice. If instructions are unclear or you don't know what they are talking about, let them know. Don't try to wing it. If you do good, they won't notice because they didn't know you had to wing it. If you fuck up, you (and your supperiors) will get fucked for it.
Shit accelerates the further it had to fall. Don't piss of anyone above your rank group.
While we are on that topic, it's usefull but not necessary to know the ranks by heart before joining. It saves you some time/stress in the first few days of bootcamp.
For the love of god, stay out of fights. There is no need to fuck up your career over any minor bullshit. Don't ever drink and drive. If you get caught you could lose your military driver's license too. Your superiors vouched for your reliablility to get you on that tank driver's course. Don't fuck your supperiors.
This is not some company, where it's competitors will gladly hire you if you are good and didn't do anything terrible. There is only one Canadian Army. This is your only chance. Act like it.