Elgg is difficult to get started with for newcomers. Here I list some points which have not been classified into full page explanation. Nevertheless they are important and useful to know.

Plugin order matters
Plugin order in the Administer > Tools Administration section is important. If your plugin is overriding a function in blog, then the blog plugin should appear above your plugin in the list.

main index page is only called when you log in or log out
The main index. page i.e elgg_installation_directory/index.php is called only when you log in or log out. After this, the start.php of the plugin is called.