So, disclaimer, but I definitely come from the more structured, genre side of things with my writing.

I think a lot of deciding what your ending will be has to do with what kind of story you want to tell. Most stories answer a question – will these two lovebirds get together? Will the characters escape the haunted house alive? Will this soldier live through this war, and if so, how will the war have changed him?

I would recommend nailing down what question your story fundamentally answers, and then figure out how your ending answers it. Of course, that’s a very simplified way of putting it, but I think starting out simple and building things out from there is how you can get the ball rolling on your story. Once you know how it ends, you know what pieces need to be in place for that ending to work … and as you look to see what pieces you need to put in place, you start to see the shape of your story as a whole.