Matthew, you are going to make me show my age. :)
My first motorcycle was a 1971 Penton 125 ISDT. I still have it, but it needs to be restored. I got it when I was 20 and it was all but new, as I bought it off a buddy going through a divorce. My parents would not let me get a bike when I was a kid, so you dad is a good guy. Be sure and treat him that way.
I started out just riding the natural terrain available to me. Round and round I went, until I had made a small loop that I could practice on. I was alone for about a year before a buddy bought a bike. Then we started going to one of the local riding parks to expand our knowledge and ability.
Don't worry about building a track, until you have many miles and laps under your belt on natural terrain. Why risk breaking a bike to learn a trick that will only get you and your dad in trouble with mom, when you need a cast?
Just be sure to get as much seat time as possible, and be sure and do whatever chores are asked of you, as gas is expensive. :nod:
Regarding your bike, there are at least 3 different 125s, which do you have? I have a TTR 125 that has a BBR 150 kit on it. It is a fun little play bike for sure. My daughter follows my grandson around with that bike.
Dirtbiking is truly a family sport. You are going to start making memories to last a lifetime.
Enjoy!!