How much can professional translators earn in a month?

How much can professional translators earn in a month? 

That depends on the volume. I also have a full-time job as a project officer, so I can only do translating after office hours and on weekends. As an illustration, a full-time translator can buy a decent car in two months. For a part-time translator, you can expect to make half or a third of that amount. Translating requires time, something that part-timers don’t have a lot of. 

Any tips on how to start a career as an interpreter or translator? 

Interpreters must have good listening skills and the ability to think very fast. After you master the fundamental skill, it’s all about networking. Always make sure that your translation is of great quality, so the good word will be spread. Advertise your ability to your peers, because people will remember you when they need the service.