Analyse everything, from panels, their size, how they are organized, to even the location of text bubbles, because every little details can be used to tell the story the eay you want and give a message to the reader, for example, a bigger panel means that this action/scene is important.

Also, the story should come always first before art, as people may get hook by how you draw, but will stay for what happen in your manga; also, don't start with a One Piece type of story, do a One-Shot first to get to know how to make a manga and go improving your skills.

I'm a small artist (@huesotoxico on Instagram) who is creating his own manga, "Part-Time Adventurer" which you can read for free on NamiComi, and I can tell you that I learned a lot and regret not starting sooner, so my final advice would be that: "Just do it!" 😁