Username field for my website

My website is a php script made with laravel. Im having a small issue that needs to be fixed. When a new user register for a new account on my website the website automatically creates a user name for them for example USER123 then the next person USER124. i do not want it to be like that. I want all new customers to create a custom made user name during registration and user name cannot be duplicated by other users and user name canot be changed by user. If you can complete this job i will hire you. Thank you.