Make sure you have CSV files contain all the information which you want to upload. If your not sure how to create CSV files, this is what you have to do.
Open your MS Excel or any other spread sheet program. In this case, I am creating CSV files which contain User Name, Password, First Name, Last Name, Email address and My School (which is a custom filed which I created)
Once you filled all your information save this file as CSV
Now browse and select your CSV file to upload all users to Moodle