Actually, you already have to be good at aikido to be allowed in to a dojo. No keiko for you! (shouted in soup-nazi-voice)
Dont worry, nobody knows how to take ukemi before they start to train. The purpose of training is to learn aikido and ukemi is a vital part of it. I have yet to see a dojo that doesn´t put a lot of effort to make sure that new members learn proper ukemi.