There are dozens of ways to make meatloaf, but the essential ingredients are ground meat, a filler, and a binder. What you use for each of these categories is totally up to you. You can use beef, turkey, pork, etc. Common fillers include bread crumbs, crushed crackers, or even rice. The filler is used to hold in moisture, hold the meat together, give some added texture, and make the meat stretch farther (you can get an extra serving or two by adding a filler). The binders include milk and egg. These work with the filler to hold the loaf together so it doesn't crumble.
There really isn't a wrong way to make meatloaf. As long as you have your ground meat, you can pull just about anything out of the pantry to mix with it. Do what you find delicious! The way I make mine is greatly influenced by how my mom makes hers.
I start with 2lbs of lean ground beef in a big bowl. I add 1 large diced onion, garlic, salt and pepper, cumin, paprika, cinnamon, parsley, basil, 2 tablespoons of mustard and ketchup, 1 egg, and 2 pieces of bread (I tear these into little pieces by hand). Sometimes I add a splash of milk, and sometimes I don't. Mix everything really well by hand. Using your bare hands is the best and easiest way to do it. Transfer to a baking dish and pat it into a loaf shape. Top the entire loaf with a generous amount of ketchup, spreading it around with a spoon. Bake at 350F for 1 hour. A loaf this size will feed the two of us for several days.
Serve with any sides you desire. I, personally, cannot make meatloaf without mashed potatoes - they have to be together! We also love it with green beans, brussel sprouts, or pickled beets.