How can I create shortcodes?

For example, when I put on the page.MD [header-1] it would fetch from somewhere a predefined HTML code with a nice header.

Also, it would be possible to create more advanced shortcodes ie.:
page.md: [header="Header Text"] or [header="Header Text","description"]?

Forgot about those advanced options, all I need is simple shortcodes as I mentioned in the first post.

Take a look at the shortcode-core plugin. In that there are several shortcode examples contained in the shortcodes/ folder.

Then take a look at the shortcode-ui plugin. This takes advantage of the shortcode-core plugin to extend it with more custom short codes. Based on these you would be able to create your own shortcode plugin.