One of the questions I hear most often from new writers is, "How do I know when my manuscript is ready to submit?"
First your work must be as polished as you know how to make it, but that's only the beginning. Find some objective readers who will look at your manuscript and give you feedback. Check SCBWI for critiquenics or writer's workshops. Another good source of writing classes can be your community college or local adult education center. Sometimes instructors will have private groups that you might be invited to join once the class is completed. Then revise again. Back to the readers, etc.
Once I complete a manuscript, I put it away for a time. A break from the work can be very useful because it gives me a new perspective and a little distance helps me be more objective with it comes to revising. This is particularly true with stories I've submitted. Anytime a manuscript is declined, I see it as an opportunity for improvement. My work is as polished as I can make it before submission, but if the story comes back; I always search for a way to make it better. As soon as I submit one story, I get to work on another. Each project develops new insights and teaches me something that I can apply to my rewrites.
When you've done all you know how to do, then it's time to "Give your story wings." It's a bit like sending your child off for their first day of school...scary? Yes! But you have to believe your story is prepared for the adventure. If it returns with a skinned knee, you fix it up and send it off again.