Can you believe that at first I had no Idea what to write about for the letter I? Imagine that. For the writer, ideas can come from anywhere. Listen to people when you're out shopping or standing in a line--maybe you catch an odd phrase or a snippet of conversation. You can make a note of it and file it away for possible future use. I have a fairly long computer file consisting of random ideas, puns, quirky expressions, along with questions beginning with "What if?" That question is a great tool to spur a writer's creativity. What if the phone rings? What if an old flame shows up at your doorstep? What if the power goes out when you're doing something critical? What if you had said what you really thought? What if he had stepped off the curb but the bus didn't stop? What if the next thing you wished for came true? Then what follows can be a whole series of consequences.
The best thing you can do as a writer is to let the ideas flow unedited. Jot them down. If you have a cell phone and you aren't home, write yourself an email. Don't say, "Oh, that's a dumb idea. I should forget about it." Collect them, string a few together, turn them inside out. Maybe one silly idea prompts another very good one.
Yes, writers have good ideas and bad ones. But don't squelch your creativity. Let the ideas flow when you have them, and later you can sort them out.