Is it normal to do not know what to say?

I do not know what to say when people talk to me. I have submitted I similar story before, and that time I said I use to turn red of shame... Now I do not get this, but I still get worried when other people that I do not use to talk, talk to me. Especially when it's someone that seems to know my shyness.
And when then tell a joke, I can't even fake a decent smile or laugh, what is very visible.
Please help me ;(