So many questions, indeed. The best way to make money? Free Agents. (the guys in the transfer list with an (F) by their price.)
Generally if you make someone 5* you are going to want him for your team, not to sell.
The first few days of a season are the best time to sell, but keep in mind that it isn't the best time to buy!
Good on you for building.. never stop building! (As you level up stuff costs more so watch your budget) But you do get more packs and lessen injuries from having nice facilities. Also you get more $ from it too.
Injuries are one of the biggest challenges to this game. Managing around them is tough the first few seasons. My advice is to have a large squad so you can rotate players out. And sell anyone who gets injured more than once. (Adding points to the physical column also helps a little against injury) Pretty much when I have 4 injuries, I sell the least important guy. Also remember 15 reds heals ANY length of injury, but only use reds when you *have* to.
There are many good Tutorials on the Forum, dig around and do a little reading - you'll learn a lot.
Good luck!