It's great to see a new player in Canada. I'm going to start by referring you to the site
FAQ section. Of particular note is a thread near the top about
Age Verification. To put it simply, you meet with a site representative
in person, provide proof that you are ages 18+ (driver's license or other government ID) and wait a couple of weeks for it to be processed. You will then get a tag under your name saying that you are age verified. This will grant you access to the sales threads on ASC as well as other Age Verified only sections. You can buy guns from other members through the classified section and can find out how to get guns from other sources such as retailers in the age verified section.
If you have any questions check the FAQ sections first. It will probably be answered there. If it isn't, search in the little bar at the top of the screen and on Google. Someone else probably had the same question at some point. As long as you take the time to do your own research the community won't be too hard on you. If it's obvious that you didn't put in any effort then you can expect to be chewed up and spat out by the community.
That's everything I can think of for now, so be responsible with your guns and I look forward to meeting you on the field.
Edit: Everyone beat me to it.