How to attach username at the end of url? Suppose I have a site www.example.com with many users in it. Now user "abc" is logged in, then the username should be attached at the end of that URL, like: www.example.com/abc

Also I have fix content on my website e.g. about us , contact us etc.
Now if I go to the "about us" content then the link should be : www.example.com/abc/contact-us
Try this: 

go here: admin/config/search/path/patterns

and add: /[user:name]

to your replacement patterns.