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

 For my first pony I  had an ex-show pony,12.2hh about 16 years, was  bombproof, had arthritis so could only walk - canter although the day we thought she was lame she bolted with me. hehe

i would say a school master or mistress, welsh sections pony's are fantastic for this if you are petite/young rider, and cobs for maybe an older teenager/novice, there are a variety, medium weight, heavy weight and light weights, both also great to learn on, my first ever pony was a welsh section x shetland and he was amazing little boy, being being 6 i wasnt very tall! you might find horses advertised as, 'strong but a safe ride schoolmaster' in my experience that has meant they just need the right bit for a bit extra control :)

hope this has helped! :)