Exercise. For a good amount of time. Jog, walk, bike, lift weights, etc - whichever suits you best. This, will help generate ideas.
Why this works, is mainly because of increased blood flow through the brain. It's the fastest way to get new ideas.
And, keep a notepad on you at all times. Writing, tidbits of notes. Now, your not likely to use all those notes. But ideas are going to starting flowing more and more often by doing this.
I do agree, with write what you know. But, what you don't know you can always do research on later. So, never think an idea is a bad idea because your knowledge is limited. When you commit to writing, your a lifelong student from that point on. Also, any ideas you don't use might be of interest to you in the future.
Have fun. ^^