Re: Some aikido terms!?!?
Aikiwiki is a good place to start, but ukemi is basically receiving technique; to learn how to move in a way that is both safe and powerfull such that if your partner/attacker falters for whatever reason, you can take control of the situation. So, per my very limited understanding, it's wise to apply just enough pressure to remain connected and be just relaxed enough to move in whatever direction nage dictates (since theoretically, nage is in a dominant position and should be able to make you move in any direction, assuming you stay connected to the situation).
As for ki breathing...I dunno...breathe in; breathe out and feel the effect on your body. Whether ki is real or imaginary though, I've seen the positive effect on training its practice can have.