Include the below line in your code before creating the browser instance and mention the path where the chromedriver exists. It should work.
System.setProperty("webdriver.chrome.driver","<path to chrome driver>\chromedriver.exe");
If your chromedriver is in the C drive under the Driver folder, then the code should be like this:
System.setProperty("webdriver.chrome.driver","C:\Driver\chromedriver.exe");