You usually begin learning it after you have the posting trot down, or at least on its way to perfection. Some trainers teach the 2-point before the sitting trot, which, IMO is not correct. IMO, a rider should learn a sitting trot first, then 2-point, then posting, but that's just me, lol. I learned these things from an older German Dressage trainer/rider and that's the order she taught me (or re-taught me, as the case may be, lol. She didn't like my foundation, so she started me over). Once you have the feel for the sitting trot, you can more easily learn to two-point, which leads to solid posting.
Anyhow, it's not difficult really, but it is tiring. You have to stand in the stirrups, relax your heels down, keep your back flat (bent at the hips), your upper body over the horse, shoulder's back but not tense, and only lightly rest your hands at the base of the horse's neck. This takes a LOT of core strength and leg muscle. You need to do all of this while keeping your knees, hips, and ankles loose, so they can absorb the bounce of the trot and your back stays level. I had a hunter trainer who made us 2-point around with a small foam ball at the small of our backs. Yeah, that was fun, lol.
To prepare for this intense workout, get some Pilates DVDs or take some Pilates classes. Get a big exercise ball and work on sit-ups, back lifts, etc. using the ball. Go to the gym or YMCA and swim laps using a kick board. Also work on the leg press machine, light weight and LOTS of reps. Have a weight trainer show you how to use the machine correctly, even if you think you know how, just to make sure you're targeting the correct muscles.
If you do the above, trust me, you'll be the best prepared beginner of your group, and your trainer will love you for it, lol. If you don't, then prepare for some serious torture and start carrying ibuprophen with you to the barn...
I do NOT have fond memories of learning the two point, either time I went through it, lol.