There is a well known writing motto, 'Write about what you know.' And if you are in the early stages of your writing career this is, I believe, essential. Not only will you have the knowledge and enthusiasm in the area but are better able to write with feeling.
After years of rejection, my real breakthrough came when I wrote a short piece for a magazine specialising in one my hobbies. With this acceptance I follwed this up with other articles and lead on to a series.
If writing non-fiction, write on subjects that interest you. If writing fiction, again make sure you have a feel for the subject and a thorough knoweldge of the characters you write about. I emphasise this because some writers write for markets which they think may be big, easy and potentially profitable. This is a mistake, particularly as the competition is likely to be all the greater.
Write about what you know and interests you and your work will stand far greater chance.