As a young adult, what is the most effective way to learn Spanish? Computer courses? Audio tapes? Textbooks?

Here are three things which I have found very helpful as I have learned French, and I'm sure they remain equally as applicable toward learning Spanish and most any other language. They are:

1. Get into the habit of regularly putting time into your study. While I was learning French, I made it a point to study it at the same time each day in the same place. Before too long, the association took hold in my brain and when it was 4:00 and I sat down into my desk I was already in study mode, which is an important mode to be in when studying!
2. Although vocabulary lists and pure grammar textbooks are good things and have their places, something more interesting to me (note: when your mind is interested, things go better) was to find the language in actual use. The ultimate source is to find an actual person or group of people to practice with. If that's not an option, If you can find a book that interests you that's written in the language or a TV channel or radio station in which to immerse yourself, that's great too. If you're not able to find any of the above, finding the language in use on the Internet is always an option. The way I found to practice reading French without having to dodge commercialism left and right was to go Wikipedia. If you want to practice reading Spanish you could always type http://es.wikipedia.org in your browser's address window.
3. This last suggestion is the commercial one, i.e., the language learning product that I think is worth the price it costs. Pimsleur language courses. You can probably find them at the bookstore in the languages section. Your public library might even have the beginner level packages which wouldn't cost you anything at all. You can find free tryout lessons by going to google and typing "Pimsleur Free Lessons" (without the quotes) and click what should still be the first link (I just checked it) from simonsays.com. From there you'll see a list of languages and you can click the one you want, which will start an audio stream.

I hope this helps. Buena suerte!