Self-editing is an essential step in the writing process. The minute you finish your first draft, you must set it aside, preferably for weeks so that you can gain some distance from your own work. Then you go back and go through it yourself, looking for all sorts of things (from Big Picture stuff like plot holes and info dumps to description and dialogue tags all the way down to grammar and spelling and typos) asking yourself all sorts of questions, the foremost of which should be: Does this make sense to a reader coming to this story for the first time?
Just as proofreading is the absolute last chance to catch things before publication, self-editing is the absolute last chance to catch things before someone other than yourself gets to read your story for the first time. Just as you want to make a good impression on the reader who buys your book, you want to make a good impression on your beta or your editor who will become your allies.
If you have a problem with basic grammar and punctuation, use the free versions of Grammarly and/or PWA to help clean your manuscript up. Better yet, find alpha readers who do not mind going through a rough manuscript (which is what a first draft is). Alpha readers are different from and come before beta readers. I will include links in comments below to explain this further.
The best guide to self-editing is this book: ‘Self-Editing for Fiction Writers’ by Renni Browne and Dave King. You can read this in one weekend (but should probably read it several times, like I did, to internalize everything). If you apply everything you learn from this little book, the quality of your writing will improve, putting you heads and shoulders above the mass of wannabes who self-publish their first draft and call it a novel.
Learning how to self-edit is a crucial step in your writing process. This is not just mere courtesy to the alphas and betas who often spend hours reading for no recompense. This is what will make you stand out from the crowd and make others see you as a serious writer, even if you are working on your very first manuscript.