Create a function

<script>
  function doSomething() {
   alert('Did something');
 }
</script>

Call the function when an event occurs on your button (in this case click event)

<input type="button" onclick="doSomething()" value="Do Something" />

Simple example.
Actually just edited and added some little functionality to stimulate your imagination on what could you do.

var flag = 1;

function myFunction() {
  if (flag === 1) {
    document.getElementById("demo").innerHTML = "Paragraph changed!";
    flag=-1;
  } else {
  document.getElementById("demo").innerHTML = "No result... again";
    flag=1;
  }
}

.box {
  display: block;
}

.button {
  background-color: #fff;
  border: 1px solid #dbdbdb
}

.button:hover {
  background-color: #dbdbdb;
  border: 1px solid #f0f
}

<div class="box">
  <h3> Example execute javascript on button click </h3>
  <div class="elements">
    <button class="button" onclick="myFunction()">Click me</button>
    <p id="demo">No result</p>
  </div>
</div>

I am 16 and I am pretty new to Web Development.
I have been writing a bunch of programs for practice and I got stuck on this one.

I made buttons in html with input tags.
So now I want to make those buttons do something in a javascript function, and nothing has worked out for me.

I’d really appreciate some sample code that I can learn from.

My html and javascript looks like this:

           function selectPrize() {
           var butn1 = document.getElementById.onclick();

           if (butn1) {

               alert("You won a new Car");

           }


        }

<input type="button" id="btn1" value="ClickMe" onclick="selectPrize()">