I have some problems with this too. Usually I think of something I'm interested in and then I say, what do I want to say about this? This can lead to an outline or just a stream of consciousness writing. Then all you have to do is organize it. Practice makes perfect when it comes to writing anything, and a lot of blogging is just letting your thoughts spill out in written form.
Motivate yourself by setting goals. If you're just starting blogging, tell yourself you want to write a page a day for a week or something like that. I'm terrible at goal setting myself but I found once I started it really worked!