How To Create Believable Places and Characters -
To create a believable plot, places, and characters is not as hard as it sounds. All you must remember is to be detailed with what you write. Don't overuse adjectives, but don't underuse them either. Develope some new technology and weapons, maybe even add some slang terms that are popular at that place in time. The possibilities are endless, so get to it!

Creating a New Alien Species -
To create a new alien race, you must first know what it is going to look like, its diet, its language(s), its technology, etc . . . But it's not as hard as it sounds. It's sometimes very fun. You can come up with a the race's name before or after you know all about it. Either way, it will turn out fine. Just remember to be very detailed when dealing with a new alien species; people want to know what they're reading about! Create colorful backgrounds and different customs (like in old sci-fi movies, a hand over the heart was a sign of saying "good-bye" or "hello"). Develope an alien dialect of English, create a special diet, or even draw your character to get to know it better.

Rules of Space-Travel -
If your characters need to fly through space, just remember one thing: keep time in mind. If your character has to travel at the speed of light for 100 million miles, it would take milleniums! Did you know that the light the sun gives off hits the earth five minutes later? The sun could be out and you would even know it! That's how "slow" lightspeed is. So, develop a new way to travel (like in Star Trek, people get "beamed" to different places, which only takes seconds) or have your plot take place within a smaller area.

Recommended Books -
World-Building, by Stephen L. Gillett and Ben Bova (Editor)
Writer's Guide to Creating a Science Fiction Universe, by George Ochoa and Jeffrey Osier
Space Travel, by Ben Bova
Aliens and Alien Societies, by Stanley Schmidt