The great news about Latin America is that if you know just one language, you’ll at least get by everywhere. The bad news is that language is not English.
If you learn as much Spanish as you can you’ll find the experience all the more rewarding. As we're sure you know, in the largest country in the continent, Brazil, they speak Portuguese. However many there speak Spanish and/or English and also you’ll find many words are very similar in Portuguese and Spanish (although the pronunciation is very different)

Even with a very basic command of the language you'll find the experience of travelling in South America more rewarding and locals are typically very sympathetic to those willing to make an effort.
Nothing beats learning with the native speakers and practising in real life situations, however you can get your skills up to scratch prior to leaving home with some 'teach yourself' resources. In our opinion the most effective language teacher in the world is Michel Thomas and his spanish course (link on the right) is a fantastic way for beginners or those with a little basic knowledge to really improve. No other language course provides such useful material in such as easy to learn way.
Learn Spanish for South America