There are to major web servers, Windows and Linux. Both Windows and Linux can use PHP, MySQL, Apache. Windows mainly used ASP. .NET scripting. So it boils down to what you want to use for a server.
The Most common and least expensive server is Linux , also known as LAMP (Linux Apache MySQL and PHP). So I recommend the following programming languages if you want to be a comprehensive developer.
CSS (Cascading Style Sheets)
Structured Query SQL (MySQL)
W3Schools has really good basic tutorials available.