Literature is a broad subject, and a lot of your success in a literature class depends on what kinds of tests and assessments your teacher gives you along with what they want you to know about a given piece of literature. There's also a lot of variance between genres (poetry and short stories are quite different, for example). So, it might not be that you're bad at literature, but rather that you're not sure what you're being asked to do?
Let me offer some general tips which would apply to any story-based narrative (a short story, novel, or a nonfiction narrative). Ask yourself the following questions. What's most important to consider first is the plot. What happens in the story? Can you summarize it? What are the key events? If you are confused about what happens in a story, it makes it harder to get to deeper analysis.
Once you have a grasp on the plot, then consider the characters. Who is the protagonist (main character)? Are there any antagonists (people who work against the main character)? From whose point of view is the story being told (or is it told from multiple points of view)? What are the characters like (what are their characteristics, in other words)- are they honest, trustworthy, cunning, deceitful, evil? Can you relate to any of them?
Once you've thought about the storyline/plot and have taken a look at the major characters, then you can start asking bigger questions about the tone and theme. The tone (created by the author's word choice, subject matter, and many other things) influences how you feel as a reader (the author's tone creates the readers mood). How does the story make you feel? Then, you can move onto the theme, which is an overarching moral or lesson (one of my English teachers taught me that the theme=the message). Beyond simple entertainment, why did the author write the story? Were they trying to teach us something?
If you start with plot, then move to characters, then move to more analytical questions like tone and theme, you'll hopefully improve. Like anything else, the more you read and practice your literature skills, the better you'll get, but it's also a good idea to make sure you know what you're teacher is looking for. If your teacher asks questions that you don't understand, talk to them before or after class, set up a meeting, or send them an e-mail for clarification. If your teacher walks you through one or two stories or one or two questions, you'll get a better idea of what they're looking for on tests and assignments.