A good manga story usually starts with a strong concept. Make the world-building detailed and immersive. And don't forget about the pacing - it should be neither too slow nor too fast.
First off, think of a fresh idea that hasn't been done to death. Then, build complex characters with clear motivations and growth arcs. Add in some unexpected twists and turns to keep readers on the edge of their seats.
You need to have a unique and engaging plot. Also, well-developed characters that readers can relate to or root for are crucial.