How to improve my writing skills

I am going to take part in the IELTS exam, and but I am not really very good at the writing part. I wanted to know how can I can improve my writing skills. Which expression or phrase should I use in my essay and which I had better not to use [1].

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[1] This sentence is a fragment. Consider adding the following: "I want to know which expression or..."

Good luck with your test!