int ENA=6;
int ENB=5;
int N1=7;
int N2=8;
int N3=9;
int N4=11;
void setup(){
//  pinMode(ENA,OUTPUT);
//  pinMode(ENB,OUTPUT);
//You do not need to call pinMode() 
//to set the pin as an output before calling analogWrite().
  pinMode(N1,OUTPUT);
  pinMode(N2,OUTPUT);
  pinMode(N3,OUTPUT);
  pinMode(N4,OUTPUT);
}
void loop(){
  analogWrite(ENA,100);//The PWM value is 0~255, 0 means always OFF, 255 means always ON. 
  analogWrite(ENB,100);
  digitalWrite(N1,HIGH);
  digitalWrite(N2,LOW);
  digitalWrite(N3,LOW);
  digitalWrite(N4,HIGH);
}