It's easy you just need to know that:

Ancient keyblade masters (thousands of them in 5 factions lead by 5 generals who were students to 1 master) go to war with eachother destroying the universe.(Game: Kingdom Hearts X) The pure hearted children of the world then rebuild universe.

A long time later two keyblade masters (The good Master Eraquis, and the evil Master Xehanort) are teaching their students (Eraquis has Aqua and Terra, Xehanort has Ventus) Xehanort splits his students heart into a dark side (Vanitas) and a light side (Ventus) in order to form the ultimate keyblade (The X blade) to unlock Kingdom Hearts. Ven, Aqua and Terra stop him and Vanitas. (Game: Birth By Sleep) and Master Xehanort posesses Terra as a last ditch effort to survive (Forming the fan named "Terranort")

Terranort wakes up on a world with amnesia in the care of a kind scientist named "Ansem" on a world called Radiant Garden (Hollow Bastion), Ventus his damaged heart fused with a little boy named Sora to heal, and Aqua gets lost in the realm of darkness (Aqua's time in the Realm of Darkness is the main storyline of the Game: KH 2.8)

Fastforward many years and a new trio of young heros (Sora, Riku, Kairi) get chosen to wield keyblades and have to stop's Posessed Terra/Xehanort who has stolen the kind scientist Ansem's identity, then ripped his heart and body into two seperate evil entities named "Ansem, Seeker of Darkness" and "Xenmas" (with his 13 henchmen, Organization 13), respectively. Both with amnesia but obsessed with finding Kingdom Hearts.

At some point Sora gets turned into a Heartless, which forms a Nobody (Roxas). Roxas works for Organization 13 but is recaptured and Sora is put to sleep in a giant machine to reintegrate him. Ansem tries to turn Kairi into a heartless and fails forming the glitch Nobody (Namine)

Namine takes Sora's memories and makes them into another person (Xion) Sora forgets all of Castle Oblivion (Game: Chain of Memories) but Xion dies.

After defeating both Terranort entities, the new trio (Sora, Kairi, Riku) and their allies (Yen Sid, Mickey Mouse, Axel, various Disney characters, etc) discover the two halves of Posessed Terra/Xehanort have reformed once again and he is forming an evil organization consisting of 13 of himselves from various time periods(Game: Dream Drop Distance), so they decide they must rescue the original trio (Ventus, Aqua, and Terra), and anyone else that could be an ally (Roxas and Namine, Xion) to stop him once and for all (Game: KH3)