SEO start with a blog? Finding content users WANT tool?

A lot of what I have read says basically learn by doing...have your own website/blog. Content is key for SEO. So my question is when starting a blog would you have your site going "live" with just your first (and only) blog post or get a handful together before you make your site "live"?

Also, what would you say is good tool (least expensive is best/not required), for beginners, to use to come up with blog questions to answer (assuming this is an informative blog)?