How do I use the AK-47 on CS:GO?

All the weapons in CS:GO have a static spray pattern. If you hold down the button and empty a clip the bullets will always come out in the same patterns. How this differs from other games is that it is -not- going to move your whole screen so all you have to do is keep the cross-hair on your target. This is why many people refer to CS:GO as a skill based FPS, you have to learn how to adjust for the spray pattern.

There is a very well written page detailing the spray patterns of all the guns on the community forums. I will however duplicate the AK47 information here since it is what the question is specifically about

The AK47 Spray Pattern:

This is the pattern of the bullets and these are found by standing still in the game, shooting the wall, and seeing where the bullets go. In order to send all of the bullets into a single location you need to reverse-echo this pattern with your mouse.

For the AK47 this is the recoil path:

People who learn the patterns can send more of the bullets to where they want, this is how they increase their 'accuracy' and get better chances at headshots as they are now sending 30 bullets in a much smaller location.

Again, I highly recommend checking out the community forums link above, and there are quite a few good series of Youtube videos of people showing how to do this, which ever way helps you learn. Eventually though you should hop into a bot game, aim at a wall and try to use these patterns to get smaller and smaller clusters of bullets.