To easily earn money, have the transfer window open to search for good free agent players with at least skill level 75 and below the age of 30. The more skills with younger age, the better. Sign them and offer them the highest wage you can. If your negotiator skills are low, the player you try to sign will reject your offer. Try re-signing him with a higher offer. For example, if the player wage is $35,000 the first time, the board will only allow you $45,000 for the player. The player will reject all offersbelow $70,000. When you re-sign him for the second time, the board will allow you more than $80,000. You can now sign him. When the player is yours, change his status to "For Sale". Big clubs will be interested in him and you can get a lot of money. If you manage a small club with a limited budget, do not keep this player. Otherwise, you will not have enough money for other players.
In Manager mode you will have a 90+ overall team. The Youth Academy in your office will have youth players of about 70 at the age of 15 or 16. You can either play with them throughout or sell them in your fourth of fifth season to earn millions of dollars.