How are dividends taxed if there is no wage income?

'Qualified' dividends have a special tax rate. Separately, you may not need to file if you and/or your household have an extremely low income.

You might want to use tax software to see what it comes up with for your situation.

The same way other income - business income, Social Security benefits, IRA withdrawals &c - is taxed. After filling out any needed forms or worksheets, you put the amount on the appropriate line on your form 1040 (lines 3a & 3b for dividends), then add up lines 1-7 to get your taxable income.

Reading the instructions is always a good idea :-)

PS: Just for example, I haven't had actual wage income (that is, reported on a W2) in maybe 15 years. I still have to file tax returns just like most people. The only difference is that I leave the wage income line blank.