1. Initially you have to create a site map. It shows how are the things going to be connect with other. from pages to articles.
2. Then find a suitable theme for your site. most of the time rest of
the Joomla CMS is available with the theme. Try to apply a theme
after building the site is not going to work most of the time.
3. Then start finding extensions you will need for your site.You will get alots of extension so choose wisely suitable one.
4. Now you have all the ingredients start building. install your
extensions and assign them as modules all over the site. its that simple
but remember keep the site map in your mind.
5. After building the site test it and then host it.