Are you storyboarding for someone else? You might find it difficult if you can't draw, because storyboarding for someone else is all about visual communication. So find what is comfortable to you, and imagine you're seeing it for the first time.
Is the composition striking? i.e. the silhouettes are distinct etc.
Is it relevant to the scene? i.e. camera tilts show who has the power etc.
Does it go from/to the previous/next shot without confusion? i.e. no cuts that move more than 180 degrees is a basic rule, though good directers work around it
A program? Microsoft paint will do you. It's not about the program, it's about the ideas you're having.