the only way for you to start writing is never think too much about "what to write" or "how to start"... i used to do the same thing back in high school where it took me more than half an hour to think before i start my writing... look, how many time you'd want to waste in an exam, or as a writer, how much paper you're going to throw away for draft and so on?
i may not that expert but at least with some experience of writing and lessons in class, i can help to enhance your writing skills. here are some tips how you can develop your writing skills;
first and foremost, try to be idealistic and creative. when you see something right in front of you, a pencil for example, try to relate whatever things that has to do with it: writing, drawing, wood, sharpener, exam, and many more.. next, try to develop or elaborate each idea, pencil is use to write, writing an essay, short stories, etc. second thing you have to consider is how to develop you story or essay...the climax for sure is the main point in your story.. it determines how people going to respond to your story, whether they like it or not. some writers are applying anti-climax technique in their writing, which is quite difference and unique.
it not the grammatically error that important, though it is, but the real thing is how you structure your sentences and words. how to shorten the sentence, but yet still understandable.. for example: "Jenny works in Singapore. She is an accountant. She owns her own apartment. She drives a Mercedes Benz" can be simplify into "Jenny works in Singapore as an accountant, owns her own apartment, and drives a Mercedes Benz. isn't that simple?? apart from that, to develop your writing skills, try to take about 10-15 minutes per day to write... eventually, you'll make it as your habit and sooner you'll find out it is so much fun.
hopefully those information and tips are helpful. i believe there are many other things you need to learn and consider to become a good writer... you will never know your ability of writing if you never try
good luck and happy writing guys!!!