You are pretty close. You have to declare the text variable outside the
scope of the lock to be able to return it:

string RemoveFromQueue() {
   string text;
   lock(lockQ ) {
      text = (string)testQ.Dequeue();
   }
   return text;
}

You don't have to call RemoveFromQueue in the PrintInfo method, you can
use testQ.Dequeue as it's protected inside the lock block.

You can't use testQ.Count that way in the loop, though. As both i and
testQ.Count are changing in the loop you will only show half of the
items. Just loop as long as there are items in the queue:

void PrintInfo() {
   lock(lockQ) {
      while (testQ.Count > 0) {
         Console.WriteLine("String=" + (string)testQ.Dequeue());
      }
   }
}

I would suggest that you put these methods, the testQ variable and the
lock variable in a class by themselves, so that the queue and the lock
variable are isolated from the rest of the code. That way the queue
can't be manipulated by any other code by mistake.