What is the best way to plan/schedule a European trip to multiple countries?

Next summer the wife and I want to do a trip to Europe. We want to see Spain, Portugal, France and Italy over a period of 10–14 days.

I have never planned a trip like this, and I do not even know where to start. We would like to do it cheaply, but we also plan to splurge a few nights, as well. Should we do it through a travel agency?

I saw a related question, but I am not traveling alone. Couchsurfing isn’t going to cut it for us. Plus, I need to have a plan. Even if I don’t end up following it, it’s necessary for my sanity.

Help!