The Bureau of Labor Statistics Occupational Handbook will help you. Go to http://www.bls.gov/oco/ocos175.htm
under "Interpreters and translators."
Here are some excerpts in answer to your questions.
Demand will remain strong for translators of the languages referred to as “PFIGS”—Portuguese, French, Italian, German, and Spanish(and the principal Asian languages—Chinese, Japanese, and Korean. In addition, current events and changing political environments, often difficult to foresee, will increase the need for persons who can work with other languages. For example, homeland security needs are expected to drive increasing demand for interpreters and translators of Middle Eastern and North African languages, primarily in Federal Government agencies.
The educational backgrounds of interpreters and translators vary. Knowing a language in addition to a native language is essential. Although it is not necessary to have been raised bilingual to succeed, many interpreters and translators grew up speaking two languages.
In high school, students can prepare for these careers by taking a broad range of courses that include English writing and comprehension, foreign languages, and basic computer proficiency. Other helpful pursuits include spending time abroad, engaging in comparable forms of direct contact with foreign cultures, and reading extensively on a variety of subjects in English and at least one other language.
Beyond high school, there are many educational options. Although a bachelor’s degree is often required, interpreters and translators note that it is acceptable to major in something other than a language. However, specialized training in how to do the work is generally required. A number of formal programs in interpreting and translation are available at colleges nationwide and through nonuniversity training programs, conferences, and courses. Many people who work as conference interpreters or in more technical areas—such as localization, engineering, or finance—have master’s degrees, while those working in the community as court or medical interpreters or translators are more likely to complete job-specific training programs.
Salaried interpreters and translators had median hourly earnings of $16.28 in May 2004. The middle 50 percent earned between $12.40 and $21.09. The lowest 10 percent earned less than $9.67, and the highest 10 percent earned more than $27.45.
Earnings depend on language, subject matter, skill, experience, education, certification, and type of employer, and salaries of interpreters and translators can vary widely. Interpreters and translators with language skills for which there is a greater demand, or for which there are relatively few people with the skills, often have higher earnings. Interpreters and translators with specialized expertise, such as those working in software localization, also generally command higher rates. Individuals classified as language specialists for the Federal Government earned an average of $71,625 annually in 2005. Limited information suggests that some highly skilled interpreters and translators—for example, high-level conference interpreters—working full time can earn more than $100,000 annually.