If you say so, how can I do?

"What should I do after hearing what you have said?" For ex: When someone have finished saying what he/she wants, listener do not what to do next, maybe what speaker have said can make the situation become more complicated or it does not use. so listener can say "Anh nói thế thì tôi không biết làm thế nào?: Hope it can do help.

You said that, I dont know how to do

I think this phrase can be said like this: "If you say that, then I have no idea." or "If you think so, what can I do then?" This phrase is used when you are trying to help someone but then they say something that upsets you or something means they don't need your help, you can say that "Anh nói thế thì tôi không biết làm thế nào." Or when someone asks for your help with their problems but their explaination is so hard to understand, you don't know what they are saying and it makes you don't know how to help them, you can say that phrase as well.