I'm completely new to digital painting and I have no idea what I'm doing. I don't know how to render things, everything just turns out as a thumbnail. I could really use any advice and critique I could get on this one.
Actually, that has a nice, spooky mood. Careful not to lose it as you add detail.
Next stop: reference. That looks like a classical Greek portico, with some Medieval European crenellated castle walls and, hey, a dome! So your basic architectural fantasy mixed bag I'd go to Google Images and get tons of different architectural images and slowly start to sharpen and refine the shapes. I'd leave much of it as loose as possible (like, not much more finished than it is now), but you will have to give the eyeballs some sharp details to look at.
I was once on the receiving end of a critique so savagely nasty, I marched straight out of class to the office and changed my major (sketchbook).