What are the basic platforms to learn web-designing? How we can start learning web-designing from beginning? What are the basic things to be kept in mind while designing sites? What are sitebuilders? How we can develop sites without doing the coding effectively? What are the best domain and hosting sites?

First, since Web design is a subjective art, there is no way to become perfect at it. Second, Web design is a broad area. It encompasses graphic design, UI design, database design, and programming. It's a rare single person who can master all aspects. So you need to more narrowly define what you want to do.

The basic thing to keep in mind is how will the targeted audience like it. Site builders generally describe firms, that will build a whole site soup to nuts.

I also recommend taking College courses on it as ScottGem states... you are unlikely to be good in every aspect much less perfect... so its wise to understand perfection doesn't exist... you find your strong points and focus on that aspect of it, and you develop a team who have their strengths in other areas.

Learn to code | Codecademy (http://www.codecademy.com) offers free instruction in HTML, CSS, JAVA and other scripting used on the web. Most websites today use a CMS (Contact Management System). There are several free open source CMS's available like Drupal, Joomla, WordPress and others. Get a low cost shared hosting account and start using these CMSs with the stuff you learn in http://www.codecademy.com (http://www.codecademy.com/).
A structured course through a college is also an excellent source of information.