How do you say these phrases in french?

I'm pretty sure you're mostly right, but I don't think you need to use comment. From what I learned in class, the use of savoir implies that you are saying "to know how to" in this case. Also, I think you could use de...rien in the second case. Rather than saying "I never know how to say anything" it would be translated more like "I know how to say nothing", which is generally the same meaning.