In setup(), start serial and choose your speed:
Serial.begin(9600);
Wherever you want to write to serial:
Serial.write("write this");
Wherever you want a newline:
Serial.println();
example
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
Serial.write("Hello World");
}
void loop() {
// put your main code here, to run repeatedly:
}