COSMOS Magazine has a great selection of science fiction, with one story appearing in their print issues and a selection of others online in their fiction section. I strongly recommend reading a few of the published examples at the link here to get a feel for it.
COSMOS invites fiction submissions for short stories of between 2,000 and 4,000 words in length. They should be literature first: well written pieces, stylistically and imaginatively executed, and polished works. A story should involve some element of science at its heart: whether a new technology, a new idea, a different society or alternate reality, and based on scientific premises, principles or possibilities. Characterisation is valued, as is a story arc. We are, in short, looking for good science fiction, and we only consider previously unpublished works.
Check out general submission guidelines here: http://www.cosmosmagazine.com/contact/submissions/ and a PDF with more detailed information here: http://www.cosmosmagazine.com/pdf/CMag_FictionGuidelines.pdf