I'm new in Magento and I'm doing a project, it has one of the requirements that "Allow customer register as Seller". But I have no idea how to do it, please help. Thanks.
Your requirement is not clear enough. If you want to create a marketplace module from scratch. You may have 2 options of differentiate a normal and seller customer
1) Create a custom customer attribute say register as seller with option Yes or no. You can fill that value giving customer as choice or by programmatically.
2) Create a new customer group as seller by this you can again differentiate by customer group Id in your application.
you can try free extension for your requirement.
Hope so it's help you.