Something with experience. You dont want to be the one trying to teach it how to behave. it should know the basic rules and should be willing to learn and try new experiences, and new things. a good hight for you, maybe a fraction bigger? something easy to make a bond with! 

but beware- horses get excited when they go to new places, dont give up on it if its hyper straight away, it will need to settle! 

goodluck :) xoxo

p.s - cobs or fells are generally good tempered! new forest too would be great! and btw you soo lucky to be getting one at 12, so am i! :) and i want one more than anything <3><3><3 (="" i="" share="" one="" with="" someone="" and="" hes="" lovely(the="" horse),="" just="" not="" quite="" mine="" :'(="" )="">