All right, so you've decided to become a speculative fiction author. Wonderful. Now what? Well, you're gonna sit right down at the ol' keyboard and...and...
The first skill you must develop as a novice author is coming up with story concepts. That sounds simple, but it is a skill which must be developed. When I first started out, I was at a loss for what to write about. Many of my early concepts (I was working in short stories at the time) were hackneyed ideas from the Good Old Days of pulp era collections which I read voraciously as a teenager. So the issue is not only coming up with ideas, but coming up with good ideas.
We do have an odd saving grace to fall back on: there are no original story concepts. Every possible story idea was thought up long ago, and has been rehashed countless times since. Star Trek? Think 'The Odyssey', by Homer. 'Lucifer's Hammer' by Niven and Pournelle? Try the Biblical flood. Conan the Barbarian? Check out the saga of Beowulf. Some of the top selling movies in theaters today can be conceptually traced back to the verbal legends of our earliest nomadic ancestors. (One wonders what the Neanderthals thought of Star Wars? But I digress.).
Another important factor is that good fiction has some tie-in to the reader which they can identify with. This might be historical (past or future), major events, major social or philosophical issues, or personal conflict - something the reader can feel which brings them emotionally into the story. This means that good story concepts are about things the reader has experienced, and can relate to.
About that: you have to be careful to balance your personal writing preferences with the interests of the public. You might think that a vividly pornographic story of Xena and Hercules playing strip poker is the cat's pajamas, but the great majority of readers will not. Unless you are going for a micro-niche audience, you need to understand the market can - and wishes to - relate to.
The point to all this is that finding concepts is as simple as looking at the world around you - if you know what to look for.
Lets say you have a broad idea of the sort of story you want to write; say, an action-adventure set on an alien world. Take a look out your window: are the city crews tearing up the street again? How about a story of a major construction project on an alien world? Check the weather: is it raining? Have your alien world deluged with seasonal monsoons, setting off a flooding crisis. Drop by the courthouse: is there a sensational murder trial going on? How about a manhunt that the characters get caught up in? There are plenty of human stories in the naked city, and if you get in the habit of plugging Tab A into Slot B, it can unleash an endless flood of story ideas. So cruise around, read the newspapers, browse the history stacks at the library: when something comes along that begs for your attention, then you have a story. If it whimpers and bribes you with chocolates, all the better. If it threatens to break your kneecaps, perhaps you should be writing Hard Boiled Detective fiction.
And if all else fails, take an established work you especially like, file the serial numbers off, and reuse the concept. You have to be careful not to go beyond the underlying premise: John Carter going to Alpha Centauri is asking for trouble; so is Dave Smith going to Mars and fighting green-skinned aliens. Now Dave Smith going to Alpha Centauri and fighting purple slime monsters - there you're in the clear. Since every concept ever spawned has been used countless times, hacking a well known piece is an easy solution to the topic problem. I am as guilty of this as any: one concept I have for a Furry Fantasy is a remake of 'Stagecoach', the John Wayne movie from the 30s. John Ford will turn over in his grave when it comes out.