If you express worry about using your rusty high school Spanish in Spain, friends will usually respond with "it will quickly come back to you with a little practice". But, if you never really spoke Spanish well in high school, you might not recall all that much.
Unless you already have a foundation in Italian or Portuguese (languages closely related to Spanish) you are unlikely to be able to learn the Spanish language in a few short weeks. You can in a couple of weeks learn a few Spanish phrases and functional words that will make your travels a little easier - see the articles I only Speak English and What to Eat in Spain. Spanish language programs and CD'scan help you develop the vocabulary you need and give you some functional Spanish language phrases useful in travel. Still, when it comes to learning Spanish there is nothing better than practicing Spanish with others and being part of a structured learning process. If you have enough time before your travel, you might want to check out the Spanish language learning opportunities at your local college.
As I relate in I only Speak English you do not need to speak Spanish to travel in Spain but knowing a few words of Spanish can make a major difference in how you experience Spain. In my experience, the Spanish people are very forgiving when it comes to foreigners speaking poor Spanish. I am certain there have been times when my Spanish attempts have generated humorous stories once I was out of sight. Never the less, Spaniards have always responded positively to my effort to use their language (even if it was just a "Hola!") and have always been willing to go out of their way to assist me. I have even had some very memorable encounters with Spaniards who only spoke Spanish. Once on a early morning beach walk I encountered an elderly man on holiday (thats "vacation" for us in the USA) who spoke no English. Exchanging greetings he engaged me in conversation about the weather, the ocean water temperature, a little info about his grandchildren (playing on the beach playground), he asked where was I from and how I was enjoying Spain. Our conversation was fragmented but we both seemed to enjoy our very friendly encounter.
Even if you do not recall much of your previous Spanish training you might be happily surprised as, I often am, at your (subconscious) hearing comprehension of the language. Sometimes, I will ask question in Spanish when there is no way I should be able to understand the answer (given in Spanish). Never the less, as the Spaniard explains the answer to me, often, I suddenly will understand what is being said. The odd part of this is most of the Spanish words in the answer I didn't understand. I think part of my high school language training (which I repeated in college) must still be lurking in my brain somewhere. I just can not access it on demand. Also, context helps alot, in someways it seems easier to understand Spanish on the street than in a class room.
I have no secret method for you to learn Spanish. Pursue the various Spanish language programs on the right; make certain you check out the Spanish for Tourists Lessons which lets you test the program for free. And. check out the Spanish vocabulary videos on this site (example: Spanish you can use at the train station).
Before my next trip to Spain (late 2013 or early 2014 is my goal) I want to become at least mildly proficient in Spanish. My plan is to give myself 9 months, I will use the Spanish language CD's and programs to build by vocabulary and then check out the local college to audit a Spanish language class. I am hoping not to have to repeat Spanish 101 for the third time. This may well be my last opportunity to become mildly conversational in Spanish.