First of all, if you're developing an MMO or want to offer multiplayer support, you're going to need to learn PHP and mySQL. In theory, you could go ASP.NET instead, but I would strongly advise against it, since Microsoft servers cost more to rent, and since PHP/mySQL is much better documented and easier for beginners to learn. If you're building a single-player game, Javascript and HTML could technically be adequate, but knowing PHP will still make your life easier, in the long run.
Although you should probably set up a LAMP stack (Linux Apache mySQL PHP) on a local network so you understand how it works, hosting your own server almost never makes sense for a production environment. You could, however, save yourself some money by developing the game on your own LAMP stack. Alternately, most shared hosts (around $80 / year) provide support for PHP and mySQL, and this would be perfectly adequate for building your game. Eventually, once you're ready to launch -- and assuming it becomes even marginally popular -- you're going to need at least a VPS, and possibly a dedicated server.
Finally, a note on Flash:
In the last few years, the popularity of mobile devices has skyrocketed, and this presents a risk for Flash games, since Apple refuses to support the plugin. If you need advanced graphics support, a more timely alternative would be HTML5. Unfortunately, this carries its own set of drawbacks: namely, since it's a long way off from official release, it's not yet universally supported, and features that are supported will vary from one browser to another. So basically, HTML5 is the future, and Flash is the past. The best option for right now? Probably Javascript; you might be surprised what JS can achieve, when properly combined with CSS, and you'll need it for AJAX functions anyway. Best of all, it's supported by virtually every device and browser.
So, in conclusion, I'm recommending that you learn HTML, CSS, PHP, mySQL and Javascript, and don't be intimidated by that variety of languages; the more you learn, the more easily you learn the rest.