How to make my plugin able to be updated from admin panel?

If my guess is right that you think you add your site URL inside the plugin’s head and it will magically update everyones plugin, then I have to tell you that wont work.

You have choices:

1. Either you host your plugin on wordpress.org and updates work pretty much without you doing anything then pushing your plugin to svn with the right stable tag in the readme.
2. You use a 3rd party code like https://github.com/YahnisElsts/plugin-update-checker to handle this, this requires you to include this code inside your plugin. If you set this up this will allow you to update your plugin from every server you like.
3. Or you install or tell your customers / users to install Github Plugin Updater and host your plugin on Github. This is even easier then hosting your plugin or wordpress.org. You just push your Plgin to Github no need to worry what is the readme. All you need it the one line inside the plugin header GitHub Plugin URI: githubusername/githubreponame and it will work. (It updates itself from the master branch when the plugin version in the header changes I think)

If your plugin should be free as in no money and open source and does nothing immoral according to wp.org staff then I would pick 1. for best exposure.