Too get a decent grade you need a strong argument with a certain degree of originality, show good analytical skills when handling the primary texts, and be able to engage with critical debate. That's a general overview; your university should provide a mark scheme to give you an idea of critera required for each grade boundary. I find them a little reductive, but they can be useful if you want to know what is required at each level.
Many people are suggesting extra reading. This certainly helps. However, you have to be able to navigate the critical field, and relate your argument to the debate - if someone has written x, but ignored y then say so. Do read around the subject, but don't just reproduce what other people have said. Alot of my critical reading goes in footnotes these days to allow me to foreground my argument. If you can use extensive footnotes then do - they are often a marker of professional scholarship. My department has very tight word counts - footnotes are excluded - so I often have to use long footnotes if I want to refer to reading that has informed the essay.