Tell us what you want to do, we can tell how to do that.
What is the serial data ? A number ?
There is a special function for that : Serial.parseInt() - Arduino Reference
The Serial.read() is a function, you must use the ()
The substring(..) operator on a string returns a string (a sub string), you can not assign that to a number that way.
PaulS replied when I was typing this, he also noticed the /n instead of \n