We are always arguing about money. How do we resolve it according to Islam?

First be clear about who is responsible for what: family maintenance is obligatory on the husband, while the wife's property and income are her own.
Many conflicts stem from expectations that were never discussed. Sit together and list income, basic maintenance, debts and savings. The husband provides maintenance according to his means (65:7); the wife is not obliged to contribute, but if she willingly helps, that is charity with great reward — and it is best recorded if it is treated as a loan. Avoid secret spending or taking on debt without discussing it. In polygyny, spending on each wife must be fair and transparent. Use the Calculator to plan a budget, and see Maintenance (nafkah) and Two-Household Finances for polygynous families.