Now that you've decided you know how to write, the next step is pretty simple: start writing! Any successful writer will tell you that writers write, and if you want to be a writer, you need to write a lot. Writing a lot gives you practice so you get better, both at the mechanics of it and at coming up with ideas, feeling comfortable with doing it, and so on. Plus, if you're not writing anything, what will you sell?
Where to get ideas? All around you. Every interaction with every person you come across, every place you go, every television show or news broadcast can be a source of inspiration. In my case, I have a niche: horses. Having a niche helps me to narrow down my choices of what to write about, but I also dance (another niche), eat, travel...you don't have to confine yourself to a niche, but you might find it helpful to capitalize on one if you can.
Tomorrow I'll start talking about where to sell your writing.