If you want to write hard sci-fi, a 5 minute glace at
this page should tell you all the DOs and DON'Ts about spaceships, with the rest of the site serving to elaborate if you care to learn more about a subject. There's also a good
essay on respecting science in science fiction.
Vonda N. McIntyre's "Pitfalls of Writing Science Fiction & Fantasy,
General Useful Information, & Other Opinionated Comments" is also neat. Read it
here or
here.
As for my own contribution, all I can say is that please, for the love of God, avoid what TVTropes calls a "planet of hats": having all members of a species be defined by a single trait. If the majority of your alien species have a single one-note culture, like
being logical or being warriors or being greedy, chances are you are doing something wrong.