So you need to play the games in order, otherwise you'll get lost.

Start by getting 1.5 HD remix on PS3. There, play KH I and Chain of Momories. DO NOT WATCH THE 358/2 DAYS CUTSCENES YET!!!

Then get 2.5 HD. Play KH II, THEN you can watch the 358/2 Days scenes, and the play Birth By Sleep, and watch the coded scenes (this is actually skipable.

Then, once KH 2.8 HD come out on PS4, you'll play Dream Drop Distance and the new chapter