To capitalize the first letter of a random string, you should follow these steps:

1. Get the first letter of the string;
2. Convert the first letter to uppercase;
3. Get the remainder of the string;
4. Concatenate the first letter capitalized with the remainder of the string and return the result;

You should use charAt() method, at index 0, to select the first character of the string.

var string = "freeCodecamp";

string.charAt(0); // Returns "f"

NOTE: charAt is preferable than using [ ] (bracket notation) as str.charAt(0) returns an empty string ('') for str = '' instead of undefined in case of ''[0].

You may use toUpperCase() method and convert the calling string to upper case.

var string = "freeCodecamp";

string.charAt(0).toUpperCase(); // Returns "F"

You may use slice() method and get the remainder of the string (from the second character, index 1, to the end of the string).

var string = "freeCodecamp";

string.slice(1); // Returns "reeCodecamp"

You should create a function that accepts a string as only argument and returns the concatenation of the first letter capitalized string.charAt(0).toUpperCase() and the remainder of the string string.slice(1).

var string = "freeCodecamp";

function capitalizeFirstLetter(str) {
  return str.charAt(0).toUpperCase() + str.slice(1);
}

capitalizeFirstLetter(string); // Returns "FreeCodecamp"

Or you may add that function to the String.prototype for using it directly on a string using the following code (so that the method is not enumerable but can be overwritten or deleted later):

var string = "freeCodecamp";

/* this is how methods are defined in prototype of any built-in Object */
Object.defineProperty(String.prototype, 'capitalizeFirstLetter', {
    value: function () {
        return this.charAt(0).toUpperCase() + this.slice(1);
    },
    writable: true, // so that one can overwrite it later
    configurable: true // so that it can be deleted later
});

string.capitalizeFirstLetter(); // Returns "FreeCodecamp"

stackoverflow - Capitalize the first letter of string in JavaScript