I wrote a response based off a "what if?" type question to your question, that does go in to detail about how one was called up for the Army. Check out my response this thread!

But in a very short answer, yes you either volunteered willingly (and from there had a much greater opportunity to choose what you did within the Army, as long as you showed promise and skill in that field career path) or were conscripted based on the year you were born / class. The only way of getting out of Military service in Nazi Germany was to either be physically handicapped in someway, mentally handicapped (but that was a worse fate than being called up - usually ending up being exterminated in some psych-ward / concentration camp), or by currently (or have been accepted in to an apprenticeship) for a vital non-military service to the Reich. An example of this was you have been accepted in to college and are getting a degree in Engineering, and thus will be of need and use in Organization Todt (Germany's big rebuilding and expanding the infrastructure works project), OR you were going to work in a coal mine. As the war progressed and the German Army needed more bodies to repopulate their decimated divisions, getting out of military service became even harder. Even within the military, getting out of combat positions was harder - by the end of the war basically all rear echelon units were combed through for fresh bodies to fill in as infantrymen.

Haha figured with your response! :P

Good luck!