Editing is a ferocious beast, best laid siege to with a careful plan.
Part of the answer depends on how you went about writing in the first place. How much structural planning did you do?
But, that aside, the very first thing to do is put your work aside for a week. Or two. Or more. (You need fresh eyes to edit your own work. If you dive in immediately, you will be too close to it.)
Next, print it out, get yourself a red pen, and read it. End to end. The pen is not to correct typos or other minor issues. It's to make up blocks that feel out of place. Doesn't flow properly. Not enough tension. Filler. (If you know other writers who will read it for you, will give feedback rather than just praise, get the same from them.)
Then and only then, is it time to make changes.
Do multiple passes. Start with the big things. Fix the story arc. Move stuff around if needed. Cut. Add. (There is no value in fixing word choice - tightening prose - at this point; everything is too liable to change, be rearranged, cut.) Check the plot points, any clues left / conflicts established and their resolutions. Do a pass for each major character, to ensure they are consisten, or evolve logically based on how they participate in the story. Check between characters, that their manerisms and speech-styles are individual.
At that point, you should have a story that basically works. Give it a few more weeks to sit before coming back for finer-level editing. (Also, get external feedback to ensure the structure really does work.) Then, you can start concentrating on word choice, on prose flavour. On tightening your writing. Do this more than once.
And, finally, if you really want to polish your work, read it backwards. A sentence/paragraph at a time. This breaks up the brain's autocorrect function, so you will spot homophones and other typos more easily.
(I warned you it was a beast.)