How do I start developing the land?

First of all, you need to have a very clear idea of what you want to end up with. Many people clear the whole property of all the big trees, not knowing exactly what will go where. This is usually regretted later, because having a few big trees can add aesthetic value, shade for horses in the hot summer sun, and a break for strong winds. Spending  a little more time at this stage of the game visualizing where everything goes would probably avoid this dilemma. If any big trees are within 100 feet of any structure, though, you might want to consider removing them. The worry during a wind storm that a big tree will hit your house, barn, or any other building is sometimes not worth the benefit of having them.

As far as shaping the land into a horse facility, count on heavy machinery like a track-hoe (for pulling out trees and brush), a bulldozer (for pushing dirt around and smoothing it out), chain saws (for limbing and cutting up trees), and lots of sweat equity. Of course, you could hire someone to perform all the menial labor of picking up tree limbs, sticks, rocks, and so on. Just be aware that the time this process usually requires consists of about half machinery, half physical labor. There is no easy way around it. The payoff from doing some of the work yourself, aside from financially, is that when the project is done and the horses are home there can be immense satisfaction in knowing that you did a lot of the work yourself.

 Land clearing...