as one author said: start your story on the first line. that is, don't start it too early or too late. another writer advised: choose a headline for your story as if it's a news item and use that headline as the starting point.
for example, instead of including the stuff about wanting somehting to eat, etc, you could just write:
nearly starving, bobby sat on the bench and bit into his beef burger. it was a dark moonless night and he was all alone on the street. a winter breeze blew over his face making him wish he had someplace to go. but being homeless, all he had was the bench and the underground. for now, this would do. just then a man joined him, sitting on the bench next to him. he also had a bag similar to bobby's from the same burger joint. the man took out his burger and taking a bite, said: i don't like beef... i like humans.
then before bobby could react, the man had jumped on him, digging his fangs in bobby's neck.
of course, you might then decide that the story shouldn't start that way. you might decide that the news headline for this story is: homeless man found murdered in mysterious manner. mad had fang marks in neck. And you might decide to start the story with the police surrounding the area and questioning people and a private investigator / detective, trying to figure out how bobby was killed.
another thing is that not all people preplan how their story will be written. some people like to outline everything, adding the meat later on. while other prefer to make the story as they write. (i'm the second type).
when writing, don't worry about the story or the structure or improving your writing. just write. revise and edit only after you've written it. at that stage, you might change the storyline, move around paragraphs, cut whole portions, and work on grammar. but if you worry about grammar or whether the story is good or not, it will never get written.
when writing, don't worry whether the story will be good or not. afterall, you don't have to show it to anyone or enter it in the contest. you can write simply for practice. if after finishing it, the story is good, you can enter it. and if it's not good, you can forget it or leave it for later revision.
Also make sure your story has lots of conflict. you'll have to find twists and turns here and there to make it longer and more interesting. conflict makes a good story.
i hope others will have more advice.
Writing exercise: write a few paragraphs for the starting scene of the above murder story.