Check empty Fields and Display red warning Message ( Javascript )

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Javascript

I have a textfield and a list which looks like this :

<form name="myform" method="post" action="register.php">

    Username : <input name="username" type="text">
    Country  : <select name="country">	  
               <option value="" selected></option>
               <option value="Afghanistan">Afghanistan</option>
               </select>

<input name="submit" type="button" value="REGISTER">
</form>

First, I want to check if there is empty field. Then, if there is empty field , display a red warning message like "Empty username" or "Empty Country" beside the textfield. So, when i press REGISTER button, it will display warning if there is any empty field. Otherwise, It will proceed to register.php

Thanks in advance!