We can't really tell you what to do.
The fact is, we don't know what tools you would rather use, or what will work better for you. I'm going to be really general here, so bear with me:
- Install a web server, or find a free web host online. You don't need to pay for one. You're still learning. Personally, I have a WAMP server running on my desktop since I don't put anything into production, and I advise you to do the same.
- Learn a server-side scripting language. Now to get down to business. You have a lot of options here. Python is said to be the easiest to learn. PHP is the most well used server-side language. Ruby is an up-coming phenom. ASP works flawlessly with it's .NET counterparts.
- Get coding! I cannot stress this enough. The only way to learn is by doing. You can't just read a book. If you want, do a few projects (e.g. a forum, a phonebook, etc.) to make it fun!
This was a very general walk-through of what you need to do. There is still the matter of learning SQL before a server-side language, and how to use a database, and all that good stuff. But that's fairly easy to figure out with Stack Overflow and Google! Until then, good luck!
A few extras to learn about after the above: