training alone isn't everything.
training is a good way of making money, but it is slow.
How you decide is up to you, but if you train purely for the money, it is best to train 1 position only, because then they train the fastest. Because of speed you also might want to use young trainees for that purpose. Besides younger players sell for more then older, so the profit will be bigger once yuo sell.
But. As I said, training isn't everything, and it takes a while.
In the meantime you can make money trough winning games.
By winning games you will attract more fans to your games, which means more income.
This income is rather fast, since every week you have a home game. So the sooner you win, the sooner your income increases.
In order to win you first need good players, and secondly good strategy.
There is tons about strategy in the forums. To get good players, you might need to sell, or even sack some of your current players and replace them with better.
For the positions you do not train it is okay to buy players who are a little older, since they are cheaper.
This way you should see your income increase in the short run, and also in the long run if you combine it with the suggested training.
You can also opt to train a little slower still, and train more positions, thats up to you.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.