One of the hardest things for many writers is deciding where to start a story. I don't mean just figuring out what's happening; you might, after all, be joining the story in the middle of some event that's already well under way. (The fleet's under attack. The symbiont and its host are in the middle of an argument about an old problem. The teenager has just blown up at his mom.) I mean figuring out at what point to open the curtain and show the reader what's happening.
It's often best to open in the middle of an action; just throw the reader into the story and let her figure out later what it all means. At the same time, too much confusion can put your reader off, so you're often juggling between getting things moving and
explaining what's happening. Learning to do this well is largely a matter of practice. But when in doubt, you're probably better off going straight into the action: shoot first, get your hero into trouble, and let 'em ask questions later.