For the full Ukrainian experience, line up to buy a train ticket at one of ticket offices (kasy) at the Central (or Southern) terminal of the Kyiv train station. The staff does not speak English, so try to practice you Ukrainian/Russian. It is a good idea to get know the train number in advance and write in on a piece of paper, together with the departure date and time. Don’t bother to learn the Ukrainian alphabet, write down the destination city name in English as well as your name. You might be asked to provide a passport when boarding. Aside from the train station, you can buy your tickets at the Central Ticket Office on Tarasa Shevchenka bulv., which accepts credit cards. 
Thanks to EURO2012 we finally have English signs almost everywhere. Visit booking.uz.gov.ua/en/ for train schedule and ticket booking.