If you're thinking of becoming a Polish translator or you are just starting out, here follows some advice.
Of course, if you want to win clients, then showing that you are qualified is important. There are many institutions that offer courses in translation, and a Google search (for example) will help you to find the most suitable one for yourself.
However, getting qualified is only half the battle. As my Grandma always says - "It's not what you know. It's who you know." Contacts, especially if you are based in Poland, are extremely important. Even being qualified, I myself had door after door remaining closed to me until I made some contacts. Try to contact other English<>Polish translators and see if they are interested in cooperating. Many will not bother to reply, jealously guarding their work and fearing the competition. Others, however, will be more open minded and will want to help and advise you. In turn, you might help them out when they have no work. Translators are better off united than divided.
Advertise. Obviously no-one's going to ask you to do a translation if they don't know you're out there. There are several good translation sites on the internet.
These sites allow you to put up your own page very simply (without having to know HTML code) for the whole world to see and inform you of jobs relating to your language pairs. Membership is free but paying a relatively small fee enhances your job prospects. And having the whole world as your market place lets a lot of potential customers know you are out there. You may check out a good example of free and an effective freelance writing advertising.
You can also have your own website created and promoted or create and promote it yourself if you have the patience to learn how. These days, being on the net is a must.
Nevertheless, don't forget to research your local market as well. Advertise locally in the Yellow Pages. It might take you a while to get the ball rolling but consistent effort will pay off sooner or later.
Rates. You will have to know how much to charge your clients. Most good sites provide statistics on the average rates the community of people registered on the site with your language combinations charge. You may want to start off with a slightly lower than average rate (but don't be too cheap!) and start charging more once you feel more confident.
Accepting Translating Jobs. Don't accept jobs that you have no hope of doing properly. Of course this is a bit of a catch 22 situation because if you never take on jobs because you don't have experience in that field then you'll never get experience in that field. Use your judgement and try to take on jobs that relate to fields of knowledge with which you are reasonably comfortable. You may dream of translating great novels but this is not the everyday reality for most translators. You may have to do a lot of time consuming background reading on certain topics, but hey, who said it was gonna be easy?
I have spent many hours reading up on translation related issues on this fascinating website. For me, one of the great plus sides of being a Polish translator is that it keeps your brain stimulated.
Polish-English Dictionaries. Obviously you'll need some good dictionaries. You can either go for the good old fashioned dictionary in print or get a CD ROM version. Different dictionaries relate to different fields and you'll find your collection growing over the years.
Of course you're also going to need a computer and a permanent internet connection. The internet obviously provides a wealth of knowledge as well as being a medium for potential clients to contact you. At some point you'll have acquire some CAT tools. CAT stands for computer assisted translation. Basically, these are programs that memorise the text you translate or provide you with glossaries and other people's translation memories in order for translation to be more consistent in terms of style and vocabulary. You may translate a sentence one way at the start of a text and translate the same page differently further down. CAT tools help avoid this with obvious quality ramifications.
Also, it's not terribly healthy to sit for hours at a computer so get a comfortable chair that allows your circulation to flow.