Just got a 3DS and Mario Kart 7 along with other games. I have some questions.

I've been playing Mario Kart for the past two hours and I'm loving it. I know there is a max of 10 coins that can be gotten per course, but to get more quickly is it easier to just play coin runners? Do you get more coins via Streetpass? Also, how do I locate my friend code? Do the amount of coins you get increase depending on how many people are on your friends list, or how many people you come into contact via StreetPass?