Rick and Morty is one of my favorite shows.
It's my very favorite!  It was created by Justin Roiland and Dan Harmon for Adult Swim on the Cartoon Network
I thought is was just Dan Harmon. Does Justin voice some of the characters?
I'm not sure.  It originated from a short parody film of Back to the Future.  "The Real Animated Adventures of Doc and Mharti" was reated by Roiland for a festival co-founded by Harmon.
I didnt know that I'll see if I can find it on youtube. How did they get their big break?
Adult Swim came to Harmon for a show idea, and he and Roiland decided to develop a show based on the short
I see. Was it popular from the start? 
It has received universal acclaim for the originality, creativity and humor