Well, I'm about the exact same as you. I just turned 13 about 2 months ago I am 5 feet even, like 100 pounds, and my size is a 8.5! Not sure about my pant size though
Anyways my setup is a Burton Hero 138 with Burton Freestyle bindings and ThirtyTwo men's Exus size 8.5. The Burton Hero is on sale at Zumiez.com too!
Burton Hero 138cm 2012 Boys Snowboard at Zumiez : PDP
Burton Freestyle Snowboard Binding (Men's) | Peter Glenn
The board I LOVE and the bindings are good! Here are links ^ the orange color matches the board and they're medium, which will fit your feet. Actually I'd be jelous, I have the black ones