Crikey Tonya. That's a massive area. Given that you can feed a family of four all year on an area 25' x 25' or less, i'd reckon go easy to start, as Mr Clint suggests. Otherwise, you're going to have a huge area of
weeds or you'll have to work very hard to keep it all going. Do you have the rainfall or the water available to keep it all well watered? A small, well tended area will produce much better and with far less effort than a large, difficult to maintain area.
However, how big you go is up to you.
For prep, first get your soil tested. Find out the Ph, and the nutrient ratios. Then you'll know what you will need to add. Different crops may like a little more of this or that, so side dressings thru the
growing season will assist. I'd also talk to a few neighbours and ask what the previous owners were doing with that area. It's obviously been used for something agricultural.
I'd till the
weeds in the area along with whatever the additives are that your soil test reveals. Good preparation is the absolute necessity for
growing vegies.
Make a plan of the beds you intend to use. Or the rows or whatever and set the garden out on paper. An area for Brassicas, one for
potatoes and other solanums like tomatoes and eggplant etc. Cucurbits like pumpkin and cucumber would have another area. Beans and other legumes can be used to enrich soil for nitrogen loving crops like corn. Or you can
plant them with the corn and allow them to grow up the stalks. Then you will have a beginning for crop rotation which will help prevent diseases and pests from getting ahead of you.
It would be good to read up on basic crop rotation principles when setting out the areas on paper. Remember also that it's best to
plant a small amount of something often rather than a huge amount in one hit. That way you get continuity od supply and if a pest or disease gets going, the losses are not so great.
Once you have a section producing well and gain experience and confidence, then you could decide if you want to expand. Have you thought of fruit
trees?
Regarding the horsemanure. I'd pile it up on the area you will be developing next, or which you are leaving 'fallow'. Huge amounts are best. (You could easily use 10 cubic yards on a 10th of that area.) Allow it to compost and when you're ready, bust it into the soil and
plant feral
plants like pumpkin or other rambling vines in it. Horse manure can carry a lot of
weed seed and is best composted. It's not that high in nutrient but is a great soil conditioner. Cucurbits love it.
In short, i'd research, plan, read and be patient. It doesn't have to happen yesterday and preparation is the key. Gotta walk before we run. Good luck :-)