What is PWM? What is the benefit of using PWM pins? Which pins can be used for PWM output?
Look at this video below around 12:48 minutes into it and follow what he does.

Now try again the square pattern with more accurate performance. Copy and paste your program here.

// Square Pattern with "PWM" is pulse width modulation. 
// Using the PWM pins allows the programmer to control the pulses sent to the motors. 
// Pins 5 and 6 are used for PWM output. 
// Darcy Chang 
// 6/6/19 
// Arduino 1.8.5 

// Set up pins
const int OnBoardLED = 13; 
int Dlay = 500; 

void setup() 
{ 
  pinMode(9,OUTPUT); // set the pin modes to output 
  pinMode(10, OUTPUT); 
  pinMode(11,OUTPUT); 
  pinMode(12,OUTPUT); 
  pinMode(OnBoardLED,OUTPUT); 
  pinMode(5,OUTPUT); 
  pinMode(6,OUTPUT); 
} 

void loop() 
{ 
  blinky(); // blink the light 
  forward(1000,120,120); // move forward 
  stopBot(1000); // briefly pause 
  turnRight(1,80); // turn at a right angle 
} 

void blinky() 
{ 
  digitalWrite(OnBoardLED, LOW); // flash on board LED 
  delay(Dlay); 
  digitalWrite(OnBoardLED, HIGH); 
  delay(Dlay); 
} 

void forward(int amt, int lWheel, int rWheel) 
{ 
  analogWrite(6,lWheel); 
  analogWrite(5,rWheel); 
  digitalWrite(9,HIGH); // make left motor move forward 
  digitalWrite(10,LOW); 
  digitalWrite(11,HIGH); // make right motor move forward 
  digitalWrite(12,LOW); 
  delay(amt); 
  stopBot(500); 
} 

void stopBot(int pause) 
{ 
  digitalWrite(9,LOW); // stop both motors 
  digitalWrite(10,LOW); 
  digitalWrite(11,LOW); 
  digitalWrite(12,LOW); 
} 

void turnRight(int dlay,int lWheelSpeed) 
{ 
  analogWrite(6,lWheelSpeed); 
  digitalWrite(9,HIGH); // make the left motor move forward 
  digitalWrite(10,LOW); 
  digitalWrite(11,LOW); // stop the right motor 
  digitalWrite(12,LOW); 
  delay(dlay); 
}

Make your robot walk around a square pattern.

// Square Pattern 
// 6/6/19 
// Arduino 1.8.5 

// Set up pins 
const int OnBoardLED = 13; 
int Dlay = 500; 

void setup() 
{ 
  pinMode(9,OUTPUT); // set the pin modes to output 
  pinMode(10, OUTPUT); 
  pinMode(11,OUTPUT); 
  pinMode(12,OUTPUT); 
  pinMode(OnBoardLED,OUTPUT); 
} 

void loop() 
{ 
  blinky(); // blink the light 
  forward(1000); // move forward 
  stopBot(1000); // briefly pause 
  turnRight(1);  // turn at a right angle 
} 

void blinky() 
{ 
  digitalWrite(OnBoardLED, LOW); // flash on board LED 
  delay(Dlay); 
  digitalWrite(OnBoardLED, HIGH); 
  delay(Dlay); 
} 

void forward(int amt) 
{ 
  digitalWrite(9,HIGH); // make left motor move forward 
  digitalWrite(10,LOW); 
  digitalWrite(11,HIGH); // make right motor move forward 
  digitalWrite(12,LOW); 
  delay(amt); 
} 

void stopBot(int pause) 
{ 
  digitalWrite(9,LOW); // stop both motors 
  digitalWrite(10,LOW); 
  digitalWrite(11,LOW); 
  digitalWrite(12,LOW); 
} 

void turnRight(double dlay) 
{ 
  digitalWrite(9,HIGH); // make the left motor move forward 
  digitalWrite(10,LOW); 
  digitalWrite(11,LOW); // stop the right motor 
  digitalWrite(12,LOW); 
  delay(dlay); 
}

This video will show you both how to install it (if it is not installed in yours or you want to add a 2nd one) and how to program it. Mr. Patton will also talk about how ultrasonic sensors work to detect objects.

Use pin # 14 for “trig” Use pin #15 for “echo” Once you get your ultrasonic sensor working, have it moving forward and when the robot detects an object at about 15 cm, it should avoid it by changing direction.