Cardiovascular fitness helps: skipping rope is my favorite because it has coordination benefits too.
To begin with, inhale as you enter and blend, exhale as you execute the technqiue. After training, you body will get syncronized to your breathing. You won't have to control it.
Takes a while for the body to get used to the activity specific requirements of Aikido. Relax, breathe, by patient, and enjoy yourself.