Answer from April Reeves: Your first horse! How exciting! I agree with you on one thing: mares. I love mares and have always had a better, all around horse with a mare than any gelding I have ridden.
Fillies are usually 4 or under, and not something I would recommend for a first horse. Look for a mare of 5 and over. Not only will their skeletal system be mature enough for you to ride consistently but their minds will also be ready for a new rider.