Hi Christian,
Check out this thread created by Pi that will contain tips to assist on your writing, as well as other preparation things that you can do

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Useful links and posts regarding select-entry school testsIf you aren't sure, the examination isn't only consisting of an essay or two, but rather there are 4 (I think?) other exams with the topics of Verbal Reasoning, Mathematics, Numerical Reasoning and Reading Comprehension (well these were the topics for the Year 8 -> 9 Examination anyway. Also keep in mind that you should be given two separate essay papers; one on Analytical Writing and one on Creative Writing. You won't be given one paper where you have to alternate between analytical and creative writing

I have sat for exams run by the same company that runs the MHS exams (Edutest) and the essays generally aren't hard to write. The prompts for a creative piece are fairly easy to understand and write about, and with an analytical piece, the article or segment of writing that you are provided to read and write on generally isn't 'hard', from my experiences anyway.
As for finding essay prompts to base your writing upon, simply Google "essay prompts" or something along the lines of, and you should be able to find lists and such that you can pick from to start writing

Good luck studying!