I went into translating by doing a distance learning course with City Uni for a year and then taking the Diploma in Translation by the Institute of Linguists. You need to be pretty much at native level in two languages before you do the course, have a degree, and you should only translate into your native language - although I chose to ignore this as my English is a bit better than my German.
The DipTrans is a very difficult & expensive exam (around 700 pounds) with a low pass rate (20%) so I would urge anyone interested in sitting it to do the prep course, which is pretty good value at 750 pounds. It is a respected professional qualification though so you do get your money's worth...
You need to specialise to make decent money - obviously everybody would like to just translate magazine articles etc., but there isn't much of a market for that kind of stuff. I went for law as I have a European law degree and did German law as part of that (as well as English law) so it was a natural step to take. As well as being a much better money-earner, specialising means that you don't have to buy dictionaries on stuff that isn't relevant to you and that your translations are easier and quicker because you end up with quite similar material - I can whizz through a contract and earn 20-30 pounds an hour that way.
There is a lot of demand for technical translations, IT, medical etc.
I don't think translating from only one language is a problem - I could work full-time if I had the time and wanted to, and I only do German. How much work you get really depends on your field and how you market yourself.