Your weekly income and expenditure can be accessed from the Finances link in the left-hand menu for your team. Here you can see the financial outcomes of playing matches and buying and selling players.
Every time you play a home match you charge spectators to watch the match. These gate receipts provide the most consistent sort of income for your team. Gate receipts are higher for competitive matches than for friendlies and are higher for higher rated teams (the home team's rating makes more of a difference than the away team's). Most of the revenue for a multiple day match comes from the first few days.
However, each time you play a match you must also pay each of the players in your squad their match fee. A player's match fee is shown on his page.
If your squad match fees are greater than your gate takings for any match then your players will forego their excess match fees so that you never make a loss from a match. You cannot end up bankrupt.