An immigration card, or more commonly known as an arrival card, is a legal document you need to fill out with your personal information and hand in to the immigration authorities when entering to foreign countries. Although there are a small number of countries who no longer require such a document due to the biometric recording system, it is still widely practiced and certainly required by law. There is nothing you should be terrified about though, as the format is rather straightforward and no tricky questions are there. What is important is that you fill it out with correct information. All the personal information have to be exactly the same as what appear on your passport. Put extra caution and attention to details especially the spelling and numbers.
Most of the times, you need to write the number of flights, ferries, buses, or any other methods of transport you took to reach the border. Keep your boarding pass safe for this reason, and certainly do not throw it away even if you have saved copy in your computer. As for the purpose of your visit, keep it simple and disregard all the ‘extra’ details of your trip such as helping friend’s charity organisation during your stay. If you go generally as a tourist, you are a tourist, end of story. And be consistent when asked at the desk because confusion is their least favourite and they can spend the next hours to ask you questions if they want to. Finally, have a black ink pen with you, all the time, and make sure it works.