I would suggest both Spanish and French. Because of colonial history, they are second after English in number of places it’s understood and spoken. (I count Portugese as similar to Spanish). Both are similar to English with their latin roots for vocabulary.
Depending on where you live in the US, you either have Mexico with Spanish or Canada with French across the border.
Together with immigrant communities, there are many other ways of getting exposure. Exposure, if possible immersion, is the best way to learn a language. It makes a huge difference how much time you will need to learn, whether you study vocabulary for 30 min. a day, plus a bit of grammar, or whether you immerse yourself for 5 hours a day.
I’ll also disagree about the possiblity of actually getting to speak the language - this is the internet! Look at wikipedia articles in the target language, find a messageboard about your favourite topic in that language (and the country). Borrow foreign audio books and paper books from the library. Rent foreign DVDs and watch first with english subtitles, original soundtrack, and then again with foreign subtitles, foreign soundtrack. Look if there is a friendship society or similar (Spain has the Institute Cervantes for promoting Spanish culture, I forgot right now what the French have; Germany has the Goethe institut.) Look at the webpage of the consulate of the country.
Look for pen pals, there are many organisations that match people. Today, there’s also email.
There’s a program called tandem partners: you look for somebody who wants to learn English and is a native speaker of your language. You set up internet with webcam and microfone, and talk to each other, half an hour you teach him English, then half an hour he teaches you French/ spanish.
Find the online editions of the countries newspapers e.g. Le Figaro and Le Monde for France. Find the websites of the main TV channels in that country - today some snippets of shows are always on the pages to watch as mpg or stream.
Be ready to be overwhelmed at first - you’ll only get one word in 30 at the beginning, esp. if you hear a lot of different accents. Natural speech is too fast, and slang is not in the vocabulary guide. Keep trying. The more you hear and read and try to talk, the better you will get.
I recommend flash cards for vocabulary, repeating them five times until you are firm, then adding new ones. Put songs on your mp3 player and sing along while doing something else. Get relax and listen to an audio book. All kinds of different things, to keep it from being boring and a chore, and to expose you to everything of the culture.
The biggest difference I would think in all romance /germanic languages that English has lost are the genders and the informal/ formal address. There’s no way around it, and no rule to it, so simply learn each word with it’s article (le, la, il, or whatever) right from the start. It doesn’t matter at first if you say “the male train” instead of “the female train” (or whatever), or if you mix up plural and singular, people will still help you find that damn train. The more you hear the more you read the more you speak, the better you will get until it comes naturally to you and you no longer think what gender that train has. (When you talk or write your native tongue, you usually don’t think about what form to use, either, because you have heard and read enough good English to get used to it).
If you decide on Spanish, Nava here is Spanish native. If you decide on French, maybe some Canadien dopers are frankophones. If you do decide on German - though that’s of use only for Germany, Austria and Luxemburg, though you then enjoy the greatest writers, Goethe and Schiller :)), contact me if you like.