My book has hit the 20,000 word mark!?

For a young adult it is not that thin. Some sites say a YA novel is about 20 000 to 50 000 (some claim 65 000) words, however, some novels, like The Hunger Games, are well over 100 000. Of course it depends on the genre of the novel.

What I recommend is that you finish the first draft first. Even the best of authors never get everything right in the first draft. After you are done, go back and edit. I am sure you will have information you want to add that is necessary to the plot. When you are done, and it's still thin, then there is nothing else you should do. A story should only be as long as it needs to be. If and when an author tries to flesh out a novel they end up cramming unnecessary information which does NOT add to the plot in any way. This will only make their work crap, and really hard to publish. Just don't force it.

You might even be surprised to see that when you finish your novel it well be well over 50 000. Finish the story first then see what you can do after you are done.

I wish all the best.

Leave it alone for a while, maybe a few weeks. When you come back cut out loads of it, anything that doesn't flow well or sounds weak; cut it out. Rewrite all the bits that you found to be weak, let a trusted friend read it (not one that will tell you it is good, one that will say "why did ** do **, and what happened to **"). 50,000 words is okay, a lot of writers in the 40's 50's 60's etc aimed for this number. The story itself is more important than the length of it, I like to read short stories that I can read all the way through just before I shut my eyes for bed. 50,000 words takes quite a few hours to read so each chapter has to leave me wanting more, you have to leave me needing to know what happens next.

I love short books. As a young adult, I can only say that an ugly cover is FAR more likely to dissuade me than the length of the book.

A story should only be as long as it needs to be. Do not force a "fleshing out". Otherwise it's like trying to strengthen your body by adding layers of unnecessary fat.

Daring writers are those who are still remembered to this day. Don't try to please the writer. Write because you want to create an amazing story, if it is good, the number of pages or words will not matter.