A joint tax return means you file one return that lists all your combined income, deductions, dependents, and credits.
If you want to file separately, then you list your income, deductions, dependents, and credits on your return only, and your spouse lists their individual income, deductions, dependents, and credits on their return only. The only piece of information about your spouse that goes on your return is their name and SSN.
Filing separately will almost always result in owing more tax, since the tax rates can be higher and certain deductions and credits are disallowed or reduced. Filing jointly almost always results in less tax owed, even if your incomes are very different. The main reason to file separately is to show a lower income for some purposes, or because you don't want to take joint/shared responsibility for your spouse's tax return because you don't trust them. (In which case, you have bigger problems.)
