Well, looking back on it, I guess I "began" my military career as a kid -- growing up on John Wayne war movies and the like. Closer to reality, I started my career as an Army ROTC cadet in college.
"Why Army?" -- Largely because the Army program (of the three programs at my college back then) offered training in which I was interested: military mountaineering (rock climbing and rappelling); ski training (both downhill and cross country); marksmanship.
I joined the ROTC program as a freshman, and earned my first stripes as a cadet sergeant after a year. Over the four years, I became a cadet SFC -- and senior year, a cadet major -- which was one of the most senior ranks in the program.
OK, so from an Active Component perspective, being a cadet major isn't much -- but recall that I spent about half my active duty career as a Major -- it took me ten years to return to the grade I'd held as a cadet! (Sounds like history repeating itself.)
I graduated with a bachelor's degree and a commission at a 2LT, Infantry.
After two years of graduate school, I reported to Infantry Officers Basic at Fort Benning -- and stayed there for Airborne training.
Once completed, my first duty station -- and the "real" start of my military career -- was Fort Bragg and the JFK Special Warfare Center where I joined the 15th Psychological Operations Battalion -- but that's another story.