[krapp]

I kind of disagree. OP needs to start by reading. Read the classics of the genre. Read trashy pulp. Read what gets published online and in magazines. Definitely read things that aren't SF. Consume ideas, themes, thoughts, motifs, tropes, find out what's been done, whose style you like, whose style you don't, and what's been done to death.

Then write, but don't write in a vacuum. Maybe join a writers' group. Maybe take some classes. I might not necessarily start thinking about 'book' right now. Books are hideously difficult to do properly, and publishing is a competitive field, and editors have wastebaskets and know how to use them (for putting almost everything they receive into). Start thinking about short stories, and if you can manage to get published in the short form, then worry about the long form.

Also be careful about rights. If you put a story online, technically it's published. Doesn't matter where - facebook, livejournal, your personal site. That can affect whether a market is willing to pick it up afterwards.