For the full Ukrainian experience, line up to buy a train ticket at one of ticket offices (kasy) at the Central/Southern Terminals 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. Aside from the train station, you can buy your tickets at the Central Ticket Office on Tarasa Shevchenko bulv. Please note that ticket offices accept only cash.
Thanks to EURO2012 we finally have English signs almost everywhere. Visit www.uz.gov.ua/en for train schedule. Ticket booking options are also available there (though it may not work sometimes). There are several Kyiv stations in the list, choose Kyiv-Pasazhyrsky (Kyiv-Pas. in the schedule) as your arrival/departure.