JSFiddle - React, Tailwind, and code Playground

HTML

Hello!<br/>
  <br/>
  Enter your first name:
  <input type="text" id="firstName" />

JavaScript

$("#firstName").keyup(function(event) {
  var tempVal = $("#firstName").val();
  $("#firstName").val(capitaliseFirstLetter(tempVal));
});

function capitaliseFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
}