Think of your first video blog as the first step on a journey - you have no idea where it will lead you!
But you do need some kind of theme; there was a bloke in his late 70s who started a video blog on getting older. Sadly he has recently died but he attracted a huge following in his last few years.
Have a look around different blogs and analyse what you like and dislike about them then copy the likes and avoid the dislikes.
Good luck!