I would like to add that the younger you are when you learn a second language, the better. After about 16, your ability to master new languages easily drops drastically because of the way the brain develops. My kids go to an elementary school that's at least 80% hispanic kids, and guess what? They're all fully bilingual. Some of their parents and grandparents are at only at beginning levels of English. Why? Partly because they speak Spanish at home (aren't immersed in English 24/7), partly because it's much harder to learn as an adult, and English is one of the more difficult languages to master as an adult.
Knowing the benefits to brain development and language skills that one gets from gaining fluency in multiple languages as young children, I've made it a priority for my kids. One went to Spanish Immersion school for kindergarten (he's not had that available since then, but hears it at school all the time, so he's learning...though sometimes it's a lot of swearing, hehe), and my other kid is just finishing her second year of dual-immersion Spanish/English school. At that age, they pick up about a new word every day, roughly equivalent to a toddler learning his/her first language. I wish I had the same opportunities as a child. I didn't start a second language until high school (took six terms of French at a neighboring college at night my freshman and sophomore years of high school), and it took a lot more work to learn than I see in my kids. I picked up a *little* Greek, living there at 16, and have been working on Spanish alongside my daughter, but she's almost hit the 'thinking in the new language' stage, while I struggle to translate the few words I understand in my mind.
I think the important thing when you learn a second language is to choose one that you'll have the most opportunities to use regularly. Practice makes perfect. For much of the US, that means Spanish, since we share a border with Mexico, and thus have the most immigrants speaking that language. For some of the Northern states, French might be a sensible choice, since you can cross the border into Quebec to immerse yourself in that language. I've heard that when you've become fluent in your second language (you've gotten to where you can immediately switch between *thinking* in one language to thinking in the other as the need arises), then subsequent language acquisition is much easier. My high school French teacher was fluent in something like 8 languages. :Q