I have a 2 year old quarter horse filly too, I have no clue when her birthday is, so I am going by January 1st. I am wanting to start her myself too this summer.
You should, if you haven't already, get her vet checked, make her her knees are closed and her teeth are good and her back is good. I am going to wait for the vets OK to start my filly. My filly is only 14hh though, the last time I measured 1-2 months ago, but she is growing fast, and will be having a growth spurt soon.
You want your horse to be physically and mentally ready.