We get hundreds of submissions for each issue. (Yay!) This means we do not read every story in the slush pile from start to finish. Sorry.
This means your first page needs to be very good, and your first paragraph needs to be very good. Grab the attention of the editors with something unique. This could be beautiful prose, a dramatic problem, a personable voice, or a fascinating world. This could be a speculative fiction genre we don't get often such as steampunk, or humorous horror. This could also be a fun mash-up of speculative genres.
Often authors waste precious real-estate on the first page with backstory, setting descriptions or world-building info-dumps. Another way of putting this: often authors start the story too early. Start your story when the story starts.
If you must include backstory, descriptions, info-dumps, etc. put them a little later. (Notice this is market-dependent; some markets like front-loaded description.)
I hate to be negative, but ... If you have weird formatting, grammar issues or other similar problems on your first page, it will count against you.
Good luck making your first page awesome!