Johneh has a good short answer for a long subject. If you have a large lot that might be managed for forestry, its best to get a Professional Forester to walk the lot with you. In many states there is a cooperative extension run by the state or county that may do it for free. If they are doing their job well, they will open up your eyes on what a professional forester can bring to the table. A nice way to get some education for free is to take a look at the back issues on the Northern Woodlands website
https://northernwoodlands.org/magaz...laHKxpJ3SoPwjSv1ji165xzocrg4lsvhoCfRgQAvD_BwE .
Every chunk of woodlands is going to over the long term grow a certain volume of wood depending on the soils, length of season and amount of sunlight. If mother nature is in charge, its going take hundreds of years to get to this optimum balance and the remaining trees may be monsters but usually battle scarred. Mother nature's approach is survival of the fittest, grow lots of small trees and let the strongest ones eventually win out and shade out all the others. Forestry can shortcut this process and yield long term sustainable forests in 50 or 60 years (depends on the tree type). They do that by thinning the forests leaving the "best" trees for the lot while taking the rest. For hardwoods, they typically thin at least twice and the wood they take out makes fine firewood. The key is person doing the thinning needs to protect the good trees. Hardwood Veneer logs are extremely valuable, sometimes thousands of dollars for the right one, but if they are damaged while thinning, they quickly drop down to sawlog grade or utility grade or ultimately firewood.
Be aware many folks want a neat firewood pile so they can neatly stuff a stove full of uniform splits, they tend to pick the nice straight trees which someday may be worth hundreds or thousands for 30 bucks of firewood. Do yourself a favor, cut the ugly trees, separate out all the uniform stuff for when the weather gets cold and then keep the "ugly wood" for times when you dont need as much heat or want and outdoor fire. I have a wood boiler in my basement and it burns hot and fast so I do not care what the wood looks like.
The problem is most folks might own a woodlot for 20 years and think its a long time. Anything done now may not show results for a period longer than the landowner may own it. Plant a hardwood stand today and its probably your grandkids who get to see it harvested. The best thing to do in the short term is "do no harm" to the trees that could become the veneer trees 50 years from now. The nice thing is opening up the woods and encouraging a canopy for shade will happen quicker, usually noticeable in 10 years. A forester can give you great guidance but with some patience you can figure out the basics. Your firewood trees are the ones with defects like broken branches, dead or dying tops, leaners and clumps of trees. Just remember, dont damage the best trees you want to keep taking out the junk.
Unless your land has never been cut, it likely prior owners "high graded" the woods by taking the best and leaving the rest. You may end up thinning 90% of the standing wood over the long run and in some cases a forester may recommend clear cutting and starting from scratch.