Now that you have mastered HTML and Javascript you probably have the urge to make pages in which you can store user data. In order to do this you will need to use a thing called CGI. As stated elsewhere on this site, Tripod.com does support CGI and it is free. CGI can be done in several different languages but perl is the preferred. Webmonkey has a very good intro tutorial to get you started and then I recommend you visit google and search for what you need to know.
After you have filled your desire to accept and store user data you should try to add more dynamic material on your pages. DHTML, which is a mixture or CSS and Javascript, is a good way to put stuff wherever you want on a page. Once again Webmonkey has a great intro tutorial on DHTML and the rest can be figured out by expirementation.
Once your page is looking pretty good you should try to make it run more efficiently. To do this cut down on file sizes by supercompressing gif's, or lowering image quality, or lowering number of images on the page, etc. Viewers hate to wait for pages to load and big pages take a long time on a 56 or 28.8 k modem, yes there are still lots of them out there. Take out unnecessary scripts and annoying gifs that have little or no point. There are plenty of places on the web with hints on speeding up load times search google and check them out.
After that there is PHP, SQL, and much more. Where you head is your choice.