Essay writing can always be improved over time, it's a skill you'll gain the more you do it. Psychology involves a lot of memorisation (knowing key studies) as well as evaluation and applications of these into real life settings.
The topics we studied for A-level AQA psychology were: social influence, attachment, memory, biopsychology, psychopathology, approaches to psychology, issues and debates in psychological & research methods. My optional A2 topics were: cognition and development, schizophrenia and addiction.
I recommend reading into some of those topics to see if the content actually interests you. It's very boring to learn if you're not interested in it. Keep in mind that there's a fair bit of biology and maths involved in psychology too. With biology it's mainly around neuroscience, the brain and nervous system. With maths it's mainly averages, graphs, measures of dispersion and some statistical tests.