I never know what to say

I feel you. I do that all the time, even with my friends but the ones I'm most comfortable with make it easier for me to talk. My advice for you, don't stress on the little things which may sound kinda mean but in the end, you won't worry about it anymore. I would recommend trying to stay calm, take a few deep breaths, and take a few seconds to think before speaking because somehow I found it to help me out and I've lived with social anxiety all my life. Good luck! I hope I helped you out.

I am so socially awkward, I'd say my best advice is just say whatever is on your mind and if it is stupid, laugh at your self, nothing screams confidence like being able to laugh at yourself :)