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.

A good place to start is to look at stories you love and see how they're structured. I actually find this very useful with movies and some TV shows. Ask yourself how they arrived at that ending, and if it was implied from the beginning. And if it was implied, ask yourself how.

You can also follow some writing formulas to get a feel for them. A common one is the "try-fail" approach. Your characters are attempting to accomplish something -- have them fail twice, and then finally succeed. The failures themselves can be very interesting. For example, Frodo tries to take the ring to Mount Doom, but runs into the Ring Wraiths. They act as an obstacle, so the path is no longer clear.

If you take that approach, in my experience the failures will often suggest the successful ending.

Once you've written the story, go back and read it through. Sometimes endings will feel jarring because there's not enough of a suggestion for them earlier on. You can write in little hints, add a dream sequence, whatever. Over time as you practice with this, you'll develop some mastery and be able to write subtler and more mind-blowing endings.

Hope that helps.