Being multilingual isn't the only skill a translator needs, but it's certainly the most important. Translators learn their languages in many different ways; many grew up in bilingual households or countries, some learned their second or third language in school and then pursued experience abroad, some took intensive language courses or worked in a foreign country for several years, and it is also quite common for translators to become freelancers after working as military or government linguists.
Almost all translators working in the U.S. have at least a Bachelor's Degree, although not necessarily in translation. As a rule, most professional translators have at least some experience working and/or living in a country where their source language or languages are spoken; many translators lived and worked in their source language country for many years, or pursued higher education in their source language(s).
In-country experience is a big asset for a translator, since translation work involves knowing not just the structure of the language to be translated, but the cultural framework that surrounds it. This isn't to say that classroom study doesn't produce excellent translators, but it's important to realize at the outset that to be a successful professional translator, you need near-native proficiency in your source language(s); if you're starting from scratch, a few semesters of part-time language class won't be enough. As a point of reference, the U.S. Government's Defense Language Institute Foreign Language Center offers a program to teach Middle Eastern languages to government employees, and the basic program involves 63 weeks of full-time study.
Many people wonder how to tell if their language skills are good enough to work as a translator. While there are various language testing services that can tell you where you stand, probably the easiest way to get a feel for your translation readiness is to translate something. Go on the web and find a legal document, newspaper article or press release in your source language, then try to translate it. Professional translators make constant use of reference materials such as print and online dictionaries, terminology databases, etc., so when you look at your practice document, don't assume that you should be able to whip out a perfect translation on the spot. The key points are: can you understand this document on both a word-for-word and a conceptual level, and can you convey its meaning in your target language?
Translators today work in almost every conceivable language pair; while the market in the United States has historically been very strong in Western European languages such as French, German, Italian and Spanish, there is an increasing (and increasingly lucrative) market for translation in Asian and Middle Eastern Languages like Hindi, Gujarati, Urdu, Thai, Chinese, Japanese, Korean, Arabic, Farsi, Pashto and Kurdish; Central and Eastern European languages like Serbian, Czech, Slovene and Macedonian; as well as the "languages of smaller diffusion" like Nepali, Hebrew or Somali.
In most language pairs, the amount of work available is proportionate to the number of translators in the language. While there is obviously a great deal of English to Spanish translation work in the U.S., there is a correspondingly large number of translators in this language combination: and while there may not be a great deal of work in Indonesian to English, there are also not many translators in this combination, resulting in a correspondingly small amount of competition for work.