@takashichea: You shouldn't be too lost, the story is fairly straightforward. You will miss some of the character/relationship development and some of the character's backstories. It's nice to see that development (rather than read about it on the episode pages), but it shouldn't be a big deal. I'll leave it up to you whether to start with the first season or not.
If if helps you decide: the main reason I started watching this series was because of the colorful art and mature-looking characters. It just happened to have a decent story to go with it.