First step in any battle is to reccognize the terrain features. If I were in your place, I'd start by getting the facts.
1. Climate. Learn about your rainfall, temperatures, sun hours, humidity, et cetera, along the year.
2. Know your slopes. Less than 5% is flat, between 5 and 15 % is workable, more than that it's orchard or wilderness type.
3. Earth composition. Pick a few jars and look at the contents of your dirt in several places. Mix your dirt with water and let it decant for 24 hours, then measure layers. Too sandy it will not hold water. Too clayey it will risk flooding.
4.
Local species. The local weeds will tell you a lot about your soil, but you have to watch it for a whole year. Which wild animals may be an issue.
5. Water. Watch for streams, potential erosion and good places for water retention. Your land is so big that you could benefit from a
pond (or several ones).
6. Winds. Learn about the dominant winds, their risks, and how to prevent them.
7. Fire hazard. Imagine yourself a flame that wants to spread and destroy everything around. Flames love dry organic matter and air, and hate open spaces and water.
8. Interesting features in place. Are there some big rocks? Fences? A well? Pathways? Find them all, and check their current state.
9. Might sound weird, but also try to learn about your surrounding community, where they
sell useful stuff, who are skilled people around in case you need to hire someone and who can assist you with the local practices. No one knows the terrain better than the local farmers. You don't have to do things like they do, but their knowledge is golden.
10. Do some 'market reseach', see what you can provide to your community better than what it currently has. Maybe you want to raise pigs, but ducks would sell better. Find out your options for selling your produce.
Once you know what you are really into, then identify what actions are required in order of importance. As Banks says, working first with the more permanent features is wise, but it depends on your budget.
Permaculture is about designing in a way that mother Nature makes the biggest part of your work. Think of a plant that thrives in flat land, but you plant it in a sloped terrain, then you have to work a lot to make that plant perform. But if instead of fighting against the nature of your plant, you form terraces out of your slope, then you are providing the conditions that your plant needs to thrive itself. You can select growing species that adapt best to your terrain or you might work on changing your terrain so it can support other species, or both. Constant watering, constant pest control and constant weed removal is fighting against Nature, so if you think you end up doing only that, then you are doing it wrong.