I have learned HTML, CSS and CSS3, but I am unable to design a complete webpage. What should I do to learn to use them to build a website?

What you learned was static. HTML and CSS are static if you do not host it on a domain.

So to make your website interactive you should host it on a domain. Furthermore, if your website is dynamic, like amazon.com or any e-commerce website (something that would require continuous updates of products/news etc.) it would be a good idea to build up a back end for your website, using Wordpress or Drupal or any other IDE maybe.

Try to use bootsnipp.com in order to make website using bootstrap framework

Practice makes a man perfect don't let your practice down.. Increase the level of understanding things.. try as much as you can and then look for answers or help if you don't get any idea.

Practice a lot trying different ways. Go for online tutorials on youtube, W3Schools best place to learn. 

I am providing a concept that your web page a building. When you see a building from out side ,its like your mock-up or whole structure . after your divide your block , and complete step by step from top to bottom.

In order to serve HTML and CSS to client browser, you will need a method of serving those pages. You will need a server. One popular tech is Apache. Today though, pages aren't served as static pages. They usually include forms which take input from the user. I suggest reading about PHP, Javascript and AJAX to build a more2.0 web site.

There are many services offering hosting and serving your files. Most popular for smaller sites is the shared-hosting (bluehost) and for web applications that you expect to grow, a great option would be cloud hosting (AWS).