I always recommend this site: http://spanish.about.com/
Having searched the Net for French courses, this site's program was BY FAR the best. I can only imagine the Spanish should be similarly good (in fact, I took a sneak peak at it, and it seems to be high quality, indeed).
Now, I was in your shoes and can tell you that you really need a lot DISCIPLINE to succeed on your own (that's the basic advantage of enrolling on a live program at some university: You ACTUALLY have to study or you're fried!!!). But if you're diligent enough, you can do it on your own.
--When you reach a basic comprehension of grammar and are armed with decent vocabulary, start reading about subjects you're familiar with (like Martina!!!!). That way you'll undertand many unfamiliar words through context, further enhancing your vocabulary.
--Read Wikipedia articles as well as BBC news. In both cases they have Spanish versions of basically the same information/content, so you can first read in English and then, having understood the content, you can give a try to the Spanish article, with a greater chance of using logic to comprehend by yourself the meaning of previously unknown words.
--Download and install the word reference toolbar. It allows to highlight any word on your browser and right-click it to be sent to a page with its definition. This is a very fast way to better understand what you're reading as well as learning new words (because it allows to comprehend them in REAL USAGE, which is always better than getting a vague dictionary definition). If for some reason, you're unable to find this app, PM me and I'll send it to you (it's a pretty small file, 2 megs at the most).
Buena suerte y saludos,