well im a manga artist and had the same problewm writing for my manga. however i found that when writing you should write about something you really love, then your passion for it will drive you to the end of the book.
different people write in different ways. some people use the area then live in for inspiration and others(like me) delve into their minds to get inspiration. when im really stuck i ask myself a describe question like " describe a nightmare" or "describe a happy place" and if i feel that i get something successfull from that open my mind and think about how i could use this to make a story for my manga from it. i have come up with a number of different stories using this method.
you could also read and take inspiration from something you love. just remeber that if you hate, for example, action stories, dont write an action story because it will come out terrible. and if you hate gore and blood and stuff, dont write horror because it wont be all that scary.
hope i helped and good luck writing :]