I am writing this article to help you learn a good chunk of Arduino in a single sitting. I wont be going into detail about libraries, just the generic Arduino language. When you first start up the Arduino IDE you will get the standard preset code:

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  // put your main code here, to run repeatedly:

}

The setup function runs only once and the loop function runs forever. Look at the following code I have written out and we will step through it bit by bit to understand it.
        #define BUTTON_1 13 
#define LED_PIN 12  

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_1, INPUT);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
              int reading = digitalRead(BUTTON_1);
  if(reading == LOW){
       digitalWrite(LED_PIN ,HIGH);
  }else{
     digitalWrite(LED_PIN ,LOW);
  }
}

Looked at the above code? Good! Don't expect to understand it straight away. First of all we define Button_1 as 13 this will be our button input pin. We then define our LED output pin as pin 12. Inside the setup() function we begin our serial bit transfer rate or BAUD as 9600. We then use pin_mode to set our BUTTON_1 as an input pin and then LED_PIN as an output pin. In the loop() function we set a new integer called reading to detect the state of BUTTON_1. If the value of reading is low then we set LED_PIN as HIGH which turns the LED that is connect to pin 12 on. If reading is not LOW then we set LED_PIN to low which turns the LED off. 

Let's talk about those DEFINE's a little more. If you set BUTTON_1 to 7, for example, then you will have to connect you button to pin 7 of your Arduino. The same goes for LED_PIN.