It's simple once you get a couple posts in. You can create a free account then you click "Submit". On the submit page you put in the title of the work and then in the URL (link) to it (I usually link directly to a deviant art page rather than directly to the .png/jpg). As for the "choose a subreddit" section, these are like forums. There are a large number of subreddits (forums) that start with "imaginary" and are dedicated to things like weapons, monsters, technology, etc. They are formatted like "imaginarytechnology", "imaginarymonsters". If you would like more help I'd be happy to help further.