I want to learn how to build a website. Where should I start?
That depends. If you wanna make just one site, I would suggest to use a content management system such as Wordpress, Joomla or Drupal. Such systems offer several templates where you can put the titel and content in.
If you wanna start making websites, as a hobby or semiprofessional, you should learn how to write html yourself. I would start with w3schools.com and later continue with the sites DanInMA posted.