someone of skill, talent, experience should probably reply or, pretty much anyone else above me as, I am very new myself
with that said, either one of two things it sounds like for me.
1. board too big. Whenever I was renting, I had a board that would spin around on me and they had my stance real narrow. One or all of those things might be the cause or.
2. you are standing STRAIGHT up and not in a 'rider stance'.
once I got myself setup with the proper gear, stance, etc, I didnt seem to have that issue anymore. This happened to me my 3rd time out and it was pretty frustration, doing the criss cross falling leaf / garland or whatever the heck they call it turns where you traverse real far left and right to the slope, when I would go to the right side the board would spin around in a circle on me. I tried shifting my weight forward, back, leaning, bending more...nothing seemed to get it.
Of course, as you ride that will happen less and less Im sure on any setup as you learn more.