Hi, welcome to the community @jaz.
First of all, try to replicate your website.
If you have a login page on your website with a username text box, password text box and a login button, then create those on your screen in AppInventor.
For tips on MySQL, see here
By this, do you mean a website with dynamic content pulled in from a database or something else?
Because apps do not work like websites.
In a website, you can change the source code, refresh the page and your site is updated.
But in an app, you cannot do so.
Android apps are generally downloaded from an app store such as Google Play Store.
From there, all further updates to the app are sent via the app store or in-app-updates.