I always answer those type of questions with this advice:
You will never be able to finish an entire novel if you cant even spend the time to brainstorm your own plot. The best advice I can give you is create a character and follow them around and see what kind of trouble they can get into. Ask yourself some questions:
-what does my character want?
-what is my character going to do to get it?
-who is going to try and stop my character and what are they going to do?
-what does my character do to over the obstacles?
Boom. instant basic plot.
Now decide what kind of story it is. If it's fantasy choose if you want them to be magical or a on a quest, create the world, and races of indigenous people/mythological beings. Choose a time period. past, present, or future.
If it's a mystery choose 1 to 3 people to cast suspicion on. decide what clues will be needed to point to the correct culprit. How does the character put them all together?
Romance: probably the easiest formula out there. Decide who is going to fall in love with whom and make it hard. OR just add romance to your novel by the same method.
historical fiction is a bit harder and will require some research on your part. Now when i say research, i don;t mean wikipedia. Go to the library or bookstore and look in the history/reference sections and decide on one or two books bout that time period and read them. A timeline of significant events would be helpful too for story planning.
ABOVE ALL: read, read, read what others have written to learn how to write yourself.
edit: adding to gerald and steve. I also keep an idea notebook. and i realize i didnt completely answer your question: my inspiration is unexplainable. they really do just sort of fall into my head. I rarely consciously sit and brainstorm for an ENTIRE idea.