put words down. make sentences. you will have to edit them later. you will not get them right the first time. count on at least half a dozen edits before it's right. if that bothers you, then you need to read more before you're ready to write. i've been writing for a very long time and i do a minimum of four edits before i even think about posting my work.
Start writing. If you can't think of the first sentence, start with the second or start with the whatever part interests you the most and work towards the beginning or the end.