I come from a long line of farmers. On my Father's side we have farmed the land for more generations than I can count. The same on 1/2 of my mother's side. For 42 of my 49 years I have lived on a farm(stead). I grew up working beside my Dad, learning from him, but I've learned even more in the last 8 years doing it completely on my own. 

These are the top things I've learned:

1. Fence posts drive and dig much easier when the ground is soft which is typical late fall thru mid-spring. Plan your fencing jobs accordingly. You will appreciate that fact only when you are trying to build fence in the middle of summer once.
2. Build the fence and infrastructure Before you get the animals. Good fences make good neighbors it's true, but they also make a happy farmer. Chasing cattle, goats or horses that bust thru a poorly built fence is no fun.
3. Every single project will cost more and take more time than you allotted. Your list will never get completed. You'll just keep adding to it. Just face it now.
4. You will always need more material than you thought. So buy an extra 2x4 or those extra t-posts. You don't have enough in your stash trust me. You're gonna need them. 
5. The old adage "measure twice, cut once" is good advice. 4 foot 8 inches is not the same as 48 inches but your tired mind will tell you it is. Normally on the last cut you need on the last 2x4 you have. Refer to #4 above. 
6. Never underestimate the stubbornness and stupidity of an animal when they go into a field you don't want them in and refuse or "can't find" the gate they just entered when you're trying to get them out. 
7. Meteorologists have No idea what the weather is gonna do. You will absolutely lose a field of fresh cut hay at least once when there's "0% chance of rain" yet a storm occurs. 
8. If your partner does not share your dream or passion, it's not going to work. You might think "I can do this without him/her" but you'll come to resent that fact. Something/someone will have to go/be given up and only You can make the decision on which it will be. 
9. Inspect what you expect. If you rely on someone else to fill the waterer, lock up the chickens, feed the critters, you better verify. It only take one night of the coop not getting locked up for a raccoon to wipe out your flock. 
10. Learn the proper way to preserve your harvest. It's heartbreaking to improperly can jars to have the seals pop in storage or food spoil. Also check those freezers every day! A freezer full of thawed meat is no fun. 
11. There is nothing like a fresh tomato you grew yourself, a slice of bread you baked yourself or the sweetness of a blackberry from your own vines. 
12. The goat, cow or pig will deliver its young on the coldest, nastiest, most inopportune time possible. Be prepared for it. 
13. You-tube is invaluable for "how to" videos. Watch and learn new skills. Learn to be self-reliant. 
14. Get yourself good gloves, hats and boots. Spend the money on quality items to make yourself comfortable, efficient and safe. 
    
    And finally...
15. No matter how absolute exhausting this lifestyle is, no matter how hot and dusty the hay field is, no matter how much you hate mucking stalls or the chicken coop... it's always better than a day stuck in a windowless office. So Live Large, Live with Passion, and Live Free