How can I add a real name field beside username to user information? I want to add a real name field beside the default username when I creat a new user in admin/people/create. How can I do this? 

I have checked the RealName module but is not doing what I need.
Simply add a textfield (or two, if you want first/lastname) to your users at admin/config/people/accounts/fields. That will allow them to enter their realname and it will be displayed on the user profile page.

And if you want to display that name in various places (like node author), that's exactly what Realname module is for. It allows to configure the displayed name based on tokens (which can either be profile fields or user fields).