Well you have to exchange friend codes, it doesn't work 1 way. Put your friend code in your profile or post it. When you input someones friend code, you have to wait for that person to input your code to become friends.
At first I was weary of being friends with strangers, (I mainly play xbox and understand weirdos out there) but the 3ds community is very friendly and pleasant. I have 13 friends and Havnt bumped into one troll or rude person yet.