So you want to write online business applications and you may need to worry about security...
If you are wedded to the idea of using Visual Studio and the rest of the Microsoft web development toolset, then ASP .NET is the intended language for that purpose. I've never written MS based web apps, so I can't help you much there. But I do know it works.
If you are going to learn Java eventually anyway, then consider getting into the Tomcat/.jsp/servlet side of things. PHP is good if you want quick scripting, but Java is typically the language of choice for enterprise class software. The tight integration of your servlets with Tomcat (since it is Java based) is helpful at times, especially when you are dealing with security issues and you are not an expert in that area.
You will of course need to be familiar with HTML, CSS, and probably JavaScript to get everything to work and look the way you want it to. And if you are not comfortable with MySQL or some other database server, you will need to be as persistence becomes a difficult problem in web apps without one.
If you are truly a beginner to all things web, you should start with a simple HTML page, then learn how to style elements using CSS. Next try a few quick JavaScript tutorials or maybe use a library like jQuery.
After you have some proficiency with this, download the Java SDK, the Eclipse Web Tool Project, and Tomcat. Install them all, in that order, and then find a tutorial for creating your first .jsp page using Ecilpse. There are lots out there.
Finally, after you understand the basics of dynamic web pages (GET and POST requests at minimum), download MySQL and install it. Find a tutorial on how to use the JDBC features of Tomcat and work with it until you know how to insert data into tables and query for a result set.
There are a lot of dots to connect, and it will take a long time. Best of luck to you.