Taking A-Level literature, without O-Level lit background.

Accept that you might be confused at the beginning.
Know that O-Level Literature is very different from A-Level Literature anyway.
If you don’t already have the talent, accept that you have to work doubly hard.
The point in the subject is to encourage critical thinking and analytical skills; don’t expect teachers to spoon-feed.
Spend your time researching, developing your ideas and building your vocabulary.
Read. Read a lot. History, greek mythology, sociology, psychology and linguistics. It’s a humanities subject.
If you are a science student and find it hard to write like those who have a natural flair, stick to a format and focus on content and analysis.
School teachers don’t always have time to deconstruct your essays and guide you step by step; find help, whether it’s peer tutoring, or an established tutor.
It is possible to go from failing to getting a distinction in 3 months! But not without working very hard and working smart.
Note that different JCs teach different texts and take different papers. Do your research beforehand.