To subscribe to a channel, it might not seem easy at first (I had to take a few time before understanding how).
While you are on a YouTube video from someone you had not subscribed yet, move your left stick to the left to show the button list down the screen. Then, continue to move your left stick to the left until you reach the last (or the first, depends on how you consider it) button, its an arrow-shaped button.
Press A to discover another list of buttons. If I remember well, the first button is the button to subscribe, so press it and Ta-Daa !! Achievement Unlocked.
By the way, in the new button list, you should be able to see the Thumbs Up/Down buttons. Click on the Thumbs Up button and another achievement ! :P