If you already are an RN, talk to an officer recruiter, though not all branches have officer recruiters. The Navy has them, and I think they have specific medical recruiters.
DO NOT tell them you want to be a corpman. It's not the same as a nurse! HM is an enlisted rate, somewhere between nurse and emergency paramedic, and while highly respected, they are not nurses and are not officers. They deploy to warzones often, with the Marines. They also deploy on ships. (I'm talking Navy)
Honestly, I'd go AF or Navy first, unless you want to deploy to a warzone for a longtime. I have complete respect for Army families and soldiers. Their deployments are long (12-15 mo) and often. Navy normally doesn't go longer than 6-7 mo, though I think only a hospital ship or maybe aircraft carrier has actual nurses. The rest have corpman only. AF normally don't deploy long either or often though they can do year remotes to Korea. Natually there are exceptions.
I know an Army nurse, she is getting ready to deploy now. She's only been in about a year.
http://www.airforce.com/opportunities/healthcare/careers/nursing/
http://www.navy.com/careers/healthcare/nursing/
http://armynursecorps.amedd.army.mil/