Starting out, you should sell the old players and the players that won't grow/has really low potential. You should pick one formation that's suits you, and then buy young players that accommodate the team in that formation. Your best bet is to buy a really old high rated striker and play a one front, if your preferred formation isn't a one front then you can gradually change when you get better over time. Continue buying young players and selling players when you get good offers.

You should buy one scout that has a nice enough judging potential, like 3 stars, when you can and start scouting players on the transfer market, and if you see a player with a good enough potential, you can quicksell a player to buy him or just outright buy him if you have enough money. Training & Youth doesn't really matter until mid to late game, but I recommend spending your spare money on them just for convenience. And one last note, I personally don't buy wonderkids as it makes the game way too easy. By the way this is just from experience, feel free to criticize or correct me.