Ashley, lots of good suggestions so far! Frank provided an excellent analysis of the pros & cons of self guided travel vs. guided tours.
One aspect of your decision that none of us have knowledge of, is what kind of a budget are you working with? That will have some bearing on which method to choose. However, I suspect there won't be a huge cost difference between travel methods; either you or the tour firm will be paying for everything in Euros, and the exchange rate will be the same in both cases.
Given the fact that this is your first trip outside the U.S., you might start by reading Europe Through The Back Door. That will give you a good idea on the realities of travel in Europe, and the difference you'll encounter there.
In "reading between the lines", I suspect that a guided tour would be the best choice for you on this first trip to Europe. I'd highly recommend the Rick Steves tours, as they're a lot of fun and an incredibly efficient way to see a lot of country in a limited time. The Guides and Drivers are awesome! Check the tours link on the home page of this site for further info.
There's no RS tour that covers both Spain and Italy. There's a very good selection of tours in Italy (one of the most popular destinations!) and several in Spain. If you want a slightly broader exposure to Europe, you could have a look at the Best of Europe 14-day tour.
If you have some time free on the weekend of Mar. 7th, you might plan a trip to Seattle to attend the Travel Festival at Rick's "headquarters" in Edmonds. It would be a good "fact finding mission", and you'll be able to speak with Guides and experts on European travel (including the tour department). There will be seminars on various travel topics and European countries, and all the Guidebooks and other travel gear are usually discounted that day. Check the link on the homepage of this site.
Good luck with your decision!