If you're looking for some Shinichi x Ran, go to the wiki and look for the hearts. If you want to understand the whole story, watch the first 3 or so of the episodes, then watch the Black Organization-related episodes. (The BO-related episodes are represented by a black man or something like that.) But if you want to understand better, watch all the manga-related episodes, and skip all the anime original episodes.
That's everything I can give you, since I also did it when I watched DC from episode 1. xD