I just started my journey as Sitecore developer.I want to create a user registration page and login page.I am totally confused where should i store the user details and how to authenticate him.
I am afraid this question is quite extensive.
Sitecore uses ASP.NET membership, role and profile providers.
As a starting place, please check this Security API Cookbook.
This will give you a really good overview of concepts and also sample code related to Sitecore User Management, authentication, authorization and user profile management.
I am sure things will be more clear to you after reading this.
Welcome to Sitecore Stack!
This sounds like you want visitors to be able to login to your site. Unless they have accounts in the Sitecore database already, you probably need to integrate using SSO or by using a user store such as Active Directory.
Sitecore has an Active Directory integration which you can download here: https://dev.sitecore.net/Downloads/Active_Directory/1_3/Active_Directory_1_3.aspx
This allows you to point at your existing Active Directory instance and use it for all your logins (including authors, if you wish).
Building the front-end
In terms of building the front-end, you might want to look at this older Extranet module on GitHub: https://github.com/markstiles/SitecoreExtranet/
It has some examples of how to build some components for an extranet site and integrate with the Sitecore API. That API hasn't changed a whole lot.
Storing your users
You can choose to store newly created users in Sitecore as real users (easy, but not always advised) or in a custom database. You'll likely want to build a membership provider implementation if you want to use the Sitecore APIs to create your users and assign roles and such.
First, have a look at this post that will help you to get started with learning Sitecore. Once you have a handle on Sitecore, you are going to have to decide what kind of membership provider you are going to use. I cannot make a valid recommendation on this, as the decision will ultimately be based on your requirements, but I do recommend that you at least create a testing/learning project and try to use Sitecore membership.
Note that you can always use a 3rd-party system for storing your users and integrate that system with Sitecore by writing your own membership provider. These docs are a little out of date, but they may help to point you in the right direction and may help you understand the concept of how to write your own membership provider.