I can't pop-up on my first shortboard?

I can catch waves easily by paddling I get speed I try to pop up, can't do it quick enough, and the wave rolls and I stand up standing still. I'm used to my 6'10 funboard using my legs. By the way I'm 13 4'10 and 90 lbs, so I'm small. My shortboard is 5'10. How do I stand up on the shortboard. I did once barely and it wasn't hard to balance. I just find popping up really hard. Any tips?