As every other aspect of the game, money management is an important skill required to be successful in the game. You need money for a lot of things from getting involved in the transfer market, meeting your expenses and developing your club's facilities. As you club grows bigger, you will have better players and staff, better facilities etc. which all require more money to be made than when your club was in it's early stages.
When you take control of a club, you are given a certain amount of money. You will soon run out of this if you do not use it wisely. You have to find a means of generating revenue more than your expenditure to be able to sustain your club at a healthy bank balance always. Your board might not be happy if you end a season at a negative bank balance and may decide to fire you from your club.
You can view your current financial details from the Finances page under the Club menu. Note that the amount that shows in your bank balance may not completely available for you to use. Some actions like bidding on a player, upgrading your facilities etc will block the amount required, and you can only use the remaining balance for further activities.
Income And Expenses
Your major sources of revenue are from the sponsor, matches, and prize money. Home matches can give you revenue based on your current stadium and other facilities, and so does neutral venue matches. Once you sign a contract with a sponsor, it can also give you additional revenue per round, from win bonuses and by achieving your season's goals. Prize money depends upon your final position in the league table, and also winning other competitions.
You have some fixed expenses to meet every round. These include wages for players and staff, facility maintenance costs etc. Keep a note of how much you spend every round and try to generate an income above that so that your club can stay afloat.