The U.S. Office of Personnel Management and the U.S. Department of State disclose salaries of federal employees on OPM.gov and State.gov, respectively. The latter website specifically contains salaries of federal foreign service employees.
Each website features pay schedules according to the General Schedule payscale, which is a federal payscale that applies to federal, civilian, white-collar employees. The GS Paygrade and the Paygrade Step determine a federal employee's base pay; there are 15 consecutive GS Paygrades and 10 consecutive Paygrade Steps.
Specific amounts of base pay for federal civilian employees vary between states, which have their own GS locality pay adjustment rates. On FederalPay.org, which provides salaries of federal civilian employees by state, the average base salary for a federal civilian employee at GS Paygrade 1 and Paygrade Step 1 is roughly $21,000 as of 2015.
On OPM.gov, the base salary for a federal employee at the same paygrade and step is approximately $18,000.