in this context they have the same meaning although people generally use anything instead of something in this way.

They can both be correct, BUT in different situations - the meaning is not the same. eg1: If you are talking very seriously about your dog's long hair, and becoming very serious about it, the other person could say 'hey, come on, we're not talking about something important here!' (= this topic isn't very important) eg2: If you are talking to your friend about many things, but just smalltalk; how are you?, the weather is not good, is it?, did you see the game last night?, etc... then another friend says "Sorry to interrupt you guys, but I .., sorry about that!" you can say 'oh, that's ok, we aren't talking about anything important.' :)

As Matt said, anything is more natural for this sentence. He wanted to tell me something important He wanted to tell me anything important (this doesn't work) Would you like something/anything to eat? Either are ok.

Both correct but the second one is better.

as per me first one is right.