That's an excellent question and one that many startups and business owners struggle with, so know you're not alone. First, you want to look at who your audience is. What would be the most helpful for them? Does your product or service require more demonstration? If so, it might be best that you start with short educational videos. At the same time you need to start with what is easiest for you to create on a consistent basis. That latter part is the key. Start with what ever you can create on a consistent basis that is exceptionally good. If that is one blog post every Wednesday then start there and grow to other content types as you get a routine down.