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Genre is probably one of the easier choices you'll have to make if at all, because it can be whatever you want it to be. The only time it makes a difference is when you're actually uploading it to a site and you need to choose a category which they require.
But let's think about this a different way, what if you don't know what kind of story to write? It's usually a good rule of thumb to choose what you want to write by what you like the best. If you like fantasy choose fantasy, if you like drama choose drama, etc. Really, genre is more of a marketing tactic, it lets you know who to advertise it too and who may want it the most. The more you think about who might be reading your story, (like, random person wise), the more you'll get closer to your proper genre.
Of course genre isn't limited to main categories either. That's why it's probably better you worry about it later. If your story has fantasy/horror/scifi/drama etc, it's best to choose which theme is more presentiment in your story. Which has the greatest effect on your story in general. Usually when something is Fantasy, it has the strongest effect on your story. If it's got horror aspects, then perhaps it will be “dark fantasy”.
There are a number of genres and sub genres out there. Sometimes your style of comic may also have an effect on it. So if you draw in a manga style it might be a good idea to say it's manga. Now some people will go in and say “but if you're not japanese it's not manga”. Which isn't true. If it's stylized enough to be manga, it's manga. Same thing with manwha or manhua, if you're aiming to draw a manwha, then it's going to be a manwha. But it's always a good idea to think ahead and say that it could also be a regular comic. Don't put all your eggs in one basket as they say.
Now you're probably wondering though, “what about the genres and sub-genres?” There are thousands of them. I can't list them all here or I'd probably go insane with grief so I'm including the list below which is relatively comprehensive. But remember, the genre your story is is whatever is it's main component. That's why it's easier to decide what it is before you start to write sometimes. But I myself, find that somewhat limiting to the writing process.
https://en.wikipedia.org/wiki/List_of_writing_genres
One last thing about genres, don't be afraid to experiment and mix match around. Things that don't seem to go together often go together the best in the strangest ways. That's why sometimes it's better not to worry about genre and let the ideas flow naturally like the morning dew!