Read science fiction magazines to learn which ones publish stories like yours. You will want to learn the different markets for so-called hard science fiction, with realistic science, and soft science fiction, which has less realistic science, as well as other types of science fiction. Look online, at a magazine or bookstore, and at the library for such magazines. You can also buy market guides that list magazines that publish science fiction short stories. Because of the diverse market for science fiction, you should be able to find magazines that match your writing and story type.
Follow the submission guidelines. Each publisher has its own guidelines regarding content, word count and when they accept submissions. A magazine editor may want you to send your entire story; others will request excerpts or just an overview. Editors may disregard submissions because the author did not following the guidelines, regardless of the quality of the piece, because of the sheer numbers of short story submissions they receive. You might find the guidelines for submission in the magazine, on the magazine's website or you may wish to call the magazine for their current guidelines.
Write a brief introductory cover letter to accompany your submission. Provide some background information regarding yourself, a brief outline of the piece you are submitting, and list publications you have previously published in or any other salient achievement. Keep the letter on topic.
Submit your science fiction short story as instructed. Although you may be anxious, wait for a response for the specified response time to elapse before contacting the magazine. If you haven't received a response by then, send a polite inquiry. If you are rejected, take heart, rejection is common, and resubmit to another magazine.
Consider attending both science fiction and comic conventions to aid you in both learning more about science fiction and in meeting other writers and even publishers. These conventions represent not only what the industry has available, but also what most interests the fans.