Writing for the first time is not a walk in the park for some one who is not a writer. Some time you want to write and you do not have words to write, your mind just gets blank.
Where do you get words to write? You want to write a description of yourself and the only words that come to your mind ends in one paragraph. Where will you get the other words to put in a sentence then a Paragraph, and a full page?
First of all, relax, remove all the detractors then act as if you are speaking to some one and write freely what comes into your mind. Pick a topic to write on, write freely then try to organize your information the next time you are writing.
Begin writing on topics that you have a lot of information about, or do your research first and put down in point form. This will help you not to lack words to write and save your time.
Write consistently so as to improve on your writing, increasing the number of word count, mastering your clarity when writing. Practice writing short blogs about anything that come to your mind. You might want to write about your school life, your career, your day in life and so forth.