Well this has probably been said a million times, however the story really tells itself. You need to understand Light's thinking pattern from the very begining to know what will happen in later installments.
About the movie, however, i've tried watching it. sure there are some things that are in the anime and manga, but other things are simply a bit overdone, or downright wrong.
click to show spoiler
After all, light doesnt tell ryuk he cant eat apples in a supermarket/convenience store it is on the road, Ryuk doesnt scream when he hears this he only argues and tells why he has to eat apples, L only says "Bang" once not once for every member of the task force, L screams before falling out of his chair in the anime and manga, not the movie, and when L and Light meet in the movie, L is eating chips, which he is never seen doing in the anime or manga
My manga references could be wrong, since ive never actually read them thoroughly, so if ive said wrong information, please correct me.