Do whichever you want to do more, don't worry about what other people want.
A shorter comic where you know the way the story will progress to the end before you start drawing is usually a good way to start, as open-ended comics can get very daunting and can turn you off from making comics before you've even seen what you can do.
You could try a one-shot comic/manga. It's basically like a short-story, but in comic/manga form. You could make it whatever genre you'd like. Usually artists who are looking to just test the waters go this route, that way they can see if they like it without making a full commitment.