Has any one chosen Alevel art history? No idea about how to write an essay

It's not a commonly offered A-level so you might not find too many doing it also - but your teacher should be able to help you in the meantime and hopefully some others doing it will find this thread and give some more advice

In general I'd expect basic essay structuring apply - intro, main body divided into paragraphs, conclusion, signpost through out the essay where you are going with your argument, use PEE (point, evidence, explain) as a basic structure for your main body paragraphs, etc. Try and put secondary literature in "conversation" with each other (e.g. "X author said Y about this piece, however Z author would disagree saying W..." or, "A author said B and C author supports this position saying D...") and yourself (for that basically a "they say, I say" approach).

That's what I tried to do for my art history module last year for the essays in the exams and it went pretty well for me, but I know A-levels can have quite specific/restrictive marking criteria so make sure to check with your teacher for feedback often!