How would you animate just the arm in animation editor?

This isn't exactly a scripting question, but how would I just animate a certain body part in the animation editor plugin? By default, the first frame is the whole body straight up and down, so when I play the animation while the player is walking, their legs and other arm stop moving. Is there some way to delete the first frame of the animation or do I have to do something else to only animate the right arm?