How does a self-employed contribute to Social Security and Medicare?

When you are self-employed and you make over $400 for the year, you are required to pay self-employment tax which is your medicare and social security tax contribution. The self-employment tax rate is 15.3% which includes 12.4% for social security tax and 2.9% for Medicare tax. Please view the TurboTax FAQ about the self-employment tax for more information.

[Edited 03/30/20|01:19 PST]

The threshold is $433 of net profit, not $600.  $433 of net profit results in $400 being subject to SE taxes.